just reading through this thread, and wondering if my phone it's different to everyone else, because my phone (Samsung S9) DOES unlock from the 'sleep' screen by saying "ok Google". (there's a setting within Google Assistant) i use the function all the time while driving, it's very useful for a completely hands free experience.
HOWEVER, I'm not convinced by the security of voice unlock (much prefer face/iris/fingerprint) so what I would prefer is to be able to use Google Assistant WITHOUT unlocking the phone. This doesn't seem to be an option at the moment, unless I've missed something somewhere