I mean it seems fully implemented to me they just wanted it to be a post-release event.
For some reason.
Which is funny since Zeraora is, judging from the listing on Serebii, literally an EX Mission and it requires beating the other 2 EX Missions first. Which were both post-release events. But the Zeraora one isnt.
Anyway my read on Garchomp Z is they wanted to make a super fast dragon type that spams draco meteor. Classic "Gen 5 meta terror". But also didn't want it to be able to spam stab earth powers. Balance, you see.