Other credit cards (American Executive Master Card, Capital One, BofA) definitely seem to help. I don't know about Amex Platinum.
An Amex Platinum account holders' credit history with Amex definitely can have an impact on the credit score of many an AU on Amex Platinum accounts.
More on AU accounts helping to "build credit" of the AUs:
https://www.forbes.com/sites/nickcle...thorized-user/