I play with a few younger players who can throw 400' consistently but can't putt that well. IMO that's why most of the long throwers don't deuce more often.
They're great at 400'+ but have a hard time with 100' and in.
I, too, play sometimes with people who can top 400', and have seen a local player overthrow a 498' hole (on a golf line, no less). But one of the reasons most of these guys aren't getting more birdies, even on a 400' wide open hole, is that you have to be accurate, too. A shot that lands 50' right or left of the basket may have the distance, but not often result in a birdie.
That said, I agree that most of those boasting of big distance are, at least, exaggerating. Of course, online, it's impossible to know who is, and who isn't.