So getting the update for the Assistant yesterday it asked me to retrain my voice model. And typically it's been the "say 'Ok Google' 3 times".
This time I saw 4 spots. First one said "Ok, Google", the second then said "Hey, Google"...
3rd was "Ok, Google" and the 4th was "Hey, Google" again.
So... we can initiate the assistant in both ways now.
Siri can be initiated by the "hey", whereas Alexa doesn't need it (just say her name). Did Google choose 'hey' to marry up to Siri ... or is it just easier to say? Okay Google does have a bit of 'cacophony' to it ...
... who knows the full reason. But, it happened.