I was this guy back when I was 18-21. (many years ago)
Some men get focused on one girl and chase her for years. (I did) Some men can't read social queues and miss on subtle hints (It happens) Some get thrown down the PuA path when they keep sucking at picking up women. This guy may very well be on this path.
Dating advice for men is absolute shit. There is a massive number of very socially inept men out there that really really struggle with approaching women. They struggle with picking up on hints. I was very much "Ash" at one point in my early 20's.
He likely thought you keeping the conversation going was showing interest and he likely thought you were interested. He likely did not recognize you because he probably hits on dozens of women each day. There are guys who do this.
I am not apologizing for the guy but I find it funny seeing it from the other side how it could be interpreted.
Glad I am out of that stage of my life honestly.