The worse part of it is that any keyboard can get compromised with account synchronisation. So, the best thing is to use offline keyboards to avoid getting hacked or compromised with keyboad. I use my keyboard offline now.
I don't really get what you mean by keyboard can be compromised by account synchronization. Also, I don't think there's any such thing as offline keyboard. To avoid losing your funds to scammers, do not install fancy or third party keyboard, only use the factory installed stock keyboard.
Maybe, he is mentioning Google keyboard which tracks all the user data knowingly or unknowingly from it's user for various purposes. I have been using Gboard for years and I try to disable all the possibilities where Google can track us which may not give complete privacy but better than letting them know everything about us.
AFAIK, Google doesn't collect user data from keyboard unless the user enabled the next word suggetion so these small tweaks can give better internet security.