No acesss to MLL for regular Plat. You do get free access to the Amex lounges (in YYZ, and YYC transborder, YEG domestic and transborder). No Amex lounge in YYC concourse A though if you're flying AC domestic.
In addition, the AMEX Platinum (Canada) card can get you into the domestic, transborder or international departure AMEX Plaza lounges at YVR.
Note, in Canada both AMEX cards (regular Platinum and Aeroplus Platinum) can get you and either one guest or family (spouse, and and 2 children below a certain age - I can not remember that threshold age) to the AMEX lounges.
Of course, I understand that a AMEX Centurion card (I am not a cardmember, but I was invited to become one) can access all lounges now accessible to AMEX Platinum (including Priority Pass) and AMEX Aeroplus Platinum lounges.
Someone has to tell me, as I do not know the answer, whether non-Canadian AMEX Platinum cards can get you into the AMEX lounges. I partially know the answer to that question - outside the YVR AMEX lounges (or at least the domestic one), it is advertised that the Hong Kong AMEX Platinum card will get the cardholder access to that lounge.