It's not as simple as Signal inspiring Facebook and Whatsapp, the sequence of events happened in reverse order.
Whatsapp started encrypting messages after significant security issues in ~2012
It was purchased by Facebook under initially some administration separation terms in ~2014?
In 2016 it added e2e encryption. If I recall this was controversial because it limits fb ad potential on users.
I guess what I'm trying to say is that the timeline seems to me to still be pointing towards <<e2e came from Whatsapp not FB as an initiative, even if FB owned Whatsapp at the time.>>
Again you're not factually wrong, but I hope my restating of the timeline makes it more clear why i I think your reverse order point doesn't tell the same story.