Flexbox has nothing to do with the responsiveness of a website so I am not sure why you are passively aggressively insulting what he has developed.
The snark doesn't make sense because you don't just get a responsible layout by using flexbox, not even close. It takes a lot of thought and work that OP may have simply decided wasn't worth it.
Simply not caring for most of the web (mobile) on a site discussing basic web technology, and one that is particularly useful for responsiveness, is very much snark worthy.