Only last week I was upgraded to a Club room at the RC San Francisco after an air con fail in my non-Club room. I had assumed the new Club room upgrade came with full Club access, and I asked to be sure—it did. The new room key clearly granted access in the elevator to the Club level, too (your key was required to access the different floors in the hotel).

In the OP’s case, I’d absolutely assume Club access was granted as part of the upgrade to the Club level room. The Club attendant was in error; the OP has Club access based on the upgrade to the Club level room. The OP should feel free to access and take advantage of the Club! If in doubt, the OP should speak to a manager and insist, but to my mind that isn’t at all necessary.

Marriott policy is that Platinums are not entitled to complimentary or SNA/suite upgrades to the RC Club level at RC properties. That isn’t the same as not allowing Club access when a RC hotel DOES at its own discretion upgrade one to the RC Club level anyway. If a RC upgrades one to a Club room, then one should get all the privileges of that Club room—including access to the Club.