That wouldn’t help because the mobile phone could still listen to keystrokes and what keys are pressed can be inferred from the keystroke sounds. (We have the reference for this somewhere in the wiki.)
Most mobile phones are snitches. Users don’t control the software running on them. Very little software freedom. Treat them accordingly. Mobile phones must be far away from any secured computer so these are unable to hear anything or make any ultrasound.
related:
(Whonix is based on Kicksecure.)
Even… Not even only microphones. Eavesdropping Risk by Speakers
The only thing that Whonix could do is add a more prominent warning in the wiki: