Of course it's a boast, and a snobbish one at that. So they can look down at oiks like me who quite enjoy KFC as a once-in-a-while treat. It's like the folk who don't have TVs posting on threads about some series or other to let everyone know that they don't have TVs
It's important to them that everyone knows they don't eat KFC, own a TV or whatever
Yes it's a strange trait. I've noticed it's done quite a lot when mentioning celebrities too, there seems to be an oddly perverse type of pride and pleasure that some people take in claiming they have no knowledge of a currently popularly known figure.
OP:
"Has everyone seen that Justin Bieber has just been arrested on drug charges?"
Respondent:
"I've never even heard of him".
Because if you were normal, and you'd not heard of him, you'd just not bother replying to that thread. It's demonstrably not within your sphere of interest, and you clearly have nothing to offer in terms of opinion.
But I suspect that these people
do know very well who (say) Justin Bieber is, but rather enjoy appearing as though they're above all of that.