From what I've gathered, Signal use was prolific among people in this administration and the past one.

I'm not surprised. My own company sends out several emails that Whatsapp can't be used as it's not secure, yet I get Whatsapp messages from leadership I work with constantly.

People ignore directives all the time. Usually out of convenience.

People have even called it out in a Whatsapp chat "hey guys, we're not supposed to use Whatsapp" and people usually ignore it.