Huh, makes you wonder if it's actually doing it on his phone or if he has a keyboard and maybe dock and monitor he attaches it to. I suppose my original comment was too broad, there was a point not too long ago when everyone wanted to replace their laptop with their phone. Samsung even let you dual boot linux from your phone with DeX
Following that story as it happened, it was all on the phone with the phone keyboard and he somehow made multiple good Neovim plugins including that very popular one (which I use in multiple configs).
neovim is probably the only sane way you could code like this on a small screen. everything works pretty much the same way it does on a desktop terminal, the only thing you have to get used to is having so many lines wrapped, and not having quick access to some characters like $ or ^, but they can just be added to the toolbar in termux
There's always an edge case. Speaking of which, here's someone who would really benefit from a hard column width limit and limited nesting that modern programmers (particularly ones using various IDEs) so carelessly violate these days.