The issue with that is that WiFi is a lot higher latency than Ethernet (even rPi Ethernet which goes over USB) and DNS is one of those things that hurts a bit with higher latency.
You’re right, but there’s nothing stopping you from installing Pi-hole in Termux, I would think, aside from having to run a DNS resolver as a normal user and doing some messy DNS and network configuration. It’d probably be easier to set up by hand than to use Pi-hole.