I and others have already replied, yes, we disagree with that statement. If you make a point of saying “it is my MIL and she is staying in my home”, both of those word choices suggest she’s not paying, because she’s family to someone in your home. The word guest doesn’t even need to enter into it.
By contrast, I can’t think of a single situation in which I really need to know if a person is paying to stay, but I have nothing to go on but the presence of the word “guest” or “customer”. As a hotel worker I have no real need to know that, and ways to figure it out if I do.
If I’m housekeeping staff, it doesn’t matter. Everyone is a guest for my purposes.
If I’m a bartender, it does matter, but all I have to ask is “what room shall I charge this to?” The room is the customer. The person is a guest.
If I’m the front desk clerk, it doesn’t matter. I know as soon as the person says “I’d like to check out of my room” that they were a paying guest, and when I swipe their card, they’re now a departing guest.
Someone won a free room in an Instagram influencer challenge? They’re not paying a dime, yet they’re entitled to all the paying-customer amenities. Guest, or not? Again, as hotel staff, I don’t actually need to know that information.
Summary: Every hotel worker who needs to differentiate paying guests from non-paying guests already has easy, contextual ways to know what’s expected. Every hotel guest knows who’s paying and who isn’t paying. The knowledge is readily available without having to browbeat people into definitely using the #1 dictionary definition of common usage and definitely not the #2A definition of another common usage.
To look at the bigger picture… every single word and spelling that you’re using now is the product of hundreds of years of people using the wrong word, culminating in a small team of lexicologists throwing up their hands and saying “fuck it, I guess it means something different now.” Every single word. That’s just how languages evolve.