While the standard AmexPlat card does give access to CO lounges, it is only when you are flying CO. Access is not provided when you're on another carrier (and would not likely apply even when CO becomes a STAR partner). So if you're flying AC, you won't be let in. The AeroAmexPlat does not give you this extended lounge access at the moment other than with the Priority Pass program, but might once CO becomes a STAR partner.

BTW when did the AmexAeroPlat add the Priority Pass benefit?