I agree with this definition.
I have had many linguistic discussions on this board in which I endeavored to make it plain that I am not a prescriptivist. I embrace dynamic change in language over time. But I lament when that change involves taking multiple words that used to have individual and precise meanings, and making them just a bunch of superfluous synonyms that don’t evoke a specific idea. Past examples have included “robbery” becoming synonymous with “theft”, and “murder” becoming synonymous with “homicide”, but there were many others.
If I’m reading a book and if someone is described as going out to the car and locking something in the trunk, I’d like to be confident in what I’m picturing. Or what if I am car shopping with my wife and I say “this time I’d like to buy a vehicle with a trunk we can lock when we give the keys to a valet”. She might say “the car we have now has a truck that locks.”
Can we eventually sort it out? Of course. But why not retain a more precise meaning? If every back of a car is a trunk, then when you really need to be specific, you are practically going to have to draw a picture.
Sounds like you should be equally annoyed that “trunk” was co-opted in the first place. “I found this is an old trunk,” she said, as she handed him a binder of receipts. “Like in an old Lincoln?” he said, confused.
eta: Point being, context matters. If you want to know more about the properties of the cargo area, describing the vehicle it’s attached to seems like a perfectly cromulent way to do it.
Back of a car? What about a Porsche 911? That is a rear-engined car, and the trunk is under the front lid!
Fair point!
In our minivan, I call that space the “cargo deck”. Long Air Force career I guess, associating with too many loadmasters.
I really should call that the trunk, simply because the word is shorter but still appropriate.
Except no one would actually say that. They’d say that they found it in the trunk of an old car.
Doesn’t matter if the vehicle in question is a hatchback, an SUV, minivan or station wagon. The area behind the back seat is called the “way back.”
Not if it’s somebody else’s wife. Since it’s not my wife, I’m going to confidently assert that minivans don’t have trunks.
This is correct.
That’s right, the way back! We definitely called it that in the family station wagon when I was a kid. I haven’t thought of that for a while but I like it.
I have an SUV and I keep the seats down permanently because the only passengers I ever have are my dogs. I even have a “cage wall” separating the front seats from the back.
It’d be pretty weird to say that my dogs ride in the trunk! 
The Humane Society would come a’knockin…
You could also call it a tailgate. I prefer calling the door + area a hatchback, though. Calling it the trunk sounds weird and wrong, and calling it the cargo area sounds oddly formal.
When I call the back of my SUV anything, I call it the “hold”, which is even shorter than “trunk” (and also pleases my SF geekiness, which associates cargo holds with space ships, rather than marine ones).
A tomato can be a fruit or a vegetable, depending on the context (botanical discussion vs. culinary). Similarly the cargo space in a minivan/SUV can be a trunk or not.
If your boss says “you need to lock the computer in your trunk,” the correct answer is “sorry, I drive a minivan and I don’t have a trunk.”
If someone says “you’re not going to carry your puppy in the trunk, are you?” you say “of course not, I’ll put the carrier in the back of my SUV.”
If your grandmother says “I’m flying in with a couple of big suitcases, would they fit in your trunk?” you say “Yes.”
With many/most sedans these days having fold-down rear seats, you could argue that there is no such space even in those cars: you could sit in half of the backseat, and reach over and fold-down the other half and reach in the back to grab the weapon.
Me: I don’t call it “the trunk”, I just call it “the back”. If I had to give it a more formal name, I suppose I’d call it the cargo area.
eta: Ninja’ed by Shoeless 
I’d call it a trunk (or “boot” in my non-US days) in all cases except for when it is the exterior, like a pickup truck.
I drive a station wagon. The space behind the back seats is used the same way in my car as it is in a sedan, it is just somewhat larger. It would seem somewhat arbitrary to make a distinction. And once a station wagon has a trunk the distinction between that and an SUV’s back space is even less compelling. I’d even call the space behind the last row of seats in a minivan a trunk. It’s a slippery slope gone mad!
Exactly what I call it, informally and formally, respectively. It’s definitely not a trunk because the word “trunk” originated from when cars had actual steamer trunks strapped to the back for storage. Eventually the “trunk” became part of the car body, but clearly, minivans, SUVs and station wagons have no such trunk-like area.
In the UK, of course, the trunk is called a boot, the word thought to originate from the stagecoach boot which was originally an exterior sideways-facing seat on the side of the coach, and later on the back, and eventually turned into a cargo area. There might be more reason over there to call a cargo area a “boot” but since “boot” has come to mean the same thing as the American “trunk” it doesn’t make a lot of sense over there, either.
I was going to ask if he would rather be right or be happy.
I’ve heard it called a trunk before. I’ve also heard it called “the back.” I presume people who use that would always refer to the last row of seats and the area in front of them as the “backseat,” while I sometimes hear “the back” mean “the backseat” in cars.
SUV’s have a “the back” or “cargo area”. Thus your wife is wrong, but people are wrong all the time and society works better if we just learn to understand people and only try correcting them once or twice before giving up.