Because what GP points out is that in China everyone uses Baidu's third party keyboard. So Signal alone enough is not enough to ensure safe communication.
This isnt a signal problem, its an android problem. I don't see how Signal could fix it, short of developing their own keyboard for their own app only but that would break a shitton of android accessibility features.
So does installing a compromised USB keyboard or a broken Logitech receiver. Why is that Signal's problem?