I would presume the TG (*A) J ticket gets you in to the EVA lounge by "law", but those at the desk have to make the call.
I was never aware of any EVA benefits for normal Amex Plat cards. They have their co-branded Amex Plat card, but that is a different animal.
It's really not a big deal either way. I think it is a bit nicer than the main TG J lounge, but you trade that for being on the wrong side of the airport. Plus watch the hours.