[schrute]

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.

[jonsolo]

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.

[hombre_fatal]

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.