It’s not really an insult. One of the main uses of flexbox is responsive layouts, so this is a valid criticism (if a bit snarky). I also am viewing this on mobile and was sad that the page was much too wide for my phone.
It would be nontrivial to make this mobile friendly and require a lot more work.
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.
So insert a “please check this site on a laptop /desktop” warning.
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.
