Delusion and gratitude are two very different things. The only thing they have in common is that they make you feel happy, but that's about it. You can be thankful for having only ONE reader without that sense of delusion. Remember how in the Twitter example, the person thought they could threaten someone with their number of followers? They thought having 60 followers made them powerful. That's delusion, and I'd guess the person wasn't even thankful toward their followers. It almost sounded like they saw the followers as their minions, haha.
Yeaaah, I wouldn't trust it, either, if it comes with a self advertisement attached to it (unless it's just in their sig or whatever). But some newcomer greetings are very genuine, and people do help others out without expecting anything but appreciation in return. Unless there are bright red flags like that self advertisement thing you mentioned, I like to give people the benefit of doubt. They might be sincere, they might be not; the point is that I don't know and should not be making assumptions nor generalizations.