It is absolutely both.
I can go along with that. I am still firmly convinced the rider is sitting on the better athlete of the pair.
Wouldn’t disagree. but the horse won’t run the course on its own any more than a race car would.
Generally true. The drug of choice for cyclists is EPO
Or at least it was the last time I paid much attention to bicycle racing. Someone may have come up with something better in the last few years.
As for smallest gap, I’m having a hard time seeing how it isn’t bowling. Unlike most sports, bowling has an absolute maximum possible score. And that absolute maximum possible score is sometimes reached by beer-league amateurs, and it’s not always reached by even the top pros. So if you had a game between a beer-league amateur and the top pro, and the amateur had a really good night and the pro had a bad one, the amateur could win.
There’s always that one horse who thumbs their nose at generalities. If they had thumbs.
More than one I’m sure 
Well, yes, but by the same token, in a two-car race a gifted amateur could beat a top F1 driver if the F1 driver crashes. Or a world-class sprinter could lose to a couch potato if he pulls a hamstring halfway.
This article helpfully touches on bowling probabilities: “The odds for a PBA bowler rolling a 300 are 460 to 1, while it’s 11,500 to 1 for the average bowler.” That’s a pretty big gap.
Maybe I can let you all know what it’s like in a few months. I’m taking a three-day racing school this November.
It won’t involve F1 cars. Normal human necks can’t handle the cornering speeds for more than a minute or two. They might have the equivalent of a junior formula car.
I also think bowling because I think that if I had started down the path of bowling as a child, I’d be a pro right now. I don’t think that for almost any other sport.
With bowling it depends on what you call ‘regular people’. The average person is not a very good bowler, nothing like the pros who expect to hit mostly strikes with probably a spare or two thrown in. But someone who bowls regularly in a league, can get pretty good compared to the pros. Doing well in league bowling for several years can be an entryway to the pros.
So it depends how you define the bowler. Most people have only bowled a few times in their life, but the rest will bowl hundreds of games a year and some of those people are close to the pro level.
My understanding is that pro lanes and the ones at your neighborhood alley are treated with oil differently. The amateur ones are biased in a way that make for higher scores. The cite below claims that it will make a 50 pin difference.
A lot of people might nominate chess as a sport with a small gap, since it’s played sitting down and millions of amateurs are pretty good at it.
Those people would be wrong. While you and I can make the same moves as Magnus Carlsen, the gap in memory, mental agility and preparation time is staggering.
Here’s a fun account of an amateur playing Carlsen (who was playing 15 games simultaneously):
Sure, but that’s true of any sport. There are some people who have only played one or two games of golf in their life, or none at all, and there are others who play a game or two every weekend, weather permitting. There are some who haven’t played a game of football, or baseball, or basketball, since their high school gym class days, and some who are in amateur leagues for those sports. The OP tells us
So we’re not comparing to people who have never even played, or who haven’t played in years.
I was only qualifying my definition. The gap is much smaller with bowling. Regular league bowlers approach the pro level and usually only lack the practice time to break through. That’s where pro bowlers come from, it’s not a major college sport that I’m aware of. You don’t find that transition into the pros with other sports. It’s not that it’s impossible, in golf for instance the pros start off as amateurs and work their way up, but far fewer golfers are one birdie away from the pros where in bowling there are many more bowlers one strike away from the pros.
While true, are we talking only about the absolute pinnacle of a sport, or just pros in general? Because there are a massive amount of professional driving leagues, and several esports racers have gotten seats in some of those cars without years of racing experience. Some of them even ended up in F1 feeder series.
I think it’s also important to appreciate that we’re not talking about a legitimately small gap between pros and regular, just the smallest gap in sports. I think we also need to be talking the difference with the top league in a sport, not a developmental league, since it’s just debate.
A good amateur bowler will be able to bowl and get beaten 99 times out of 100 by the pro. A good amateur golfer will fail to make the cut 99 times out of 100. A good amateur basketball player is going to be clowned by an NBA player. A good amateur football player will be lucky to leave the field walking. A good amateur driver will need to get off the F1 or NASCAR track before he kills someone.
I totally agree and bowling is probably the best choice to the question posed in the OP. I was just pointing out that the perceived gap is larger than one might think. A 300 game in a local league isn’t nearly the same accomplishment as one in a pro tournament.