Thanks again @hpk and @link2xt for the advice about the workaround and for the update about response from the DC team. It is good to know that DC team will discuss the issues raised here.
Maybe I phrased this incorrect. What I mean is that FCM integration adds another method for correlating accounts which did not exist before. This method is trivial to exploit if device tokens are not encrypted on chatmail servers or if the encryption does not use a unique nonce for each account, and harder but not impossible to exploit if device tokens are encrypted with a unique nonce.
But you are correct this is not the only method for correlating accounts. If I understand correctly the other methods for correlating accounts can be mitigated with some current methods like disabling background sync on platforms which support this (hopefully available to all platforms in future) or in the future if DC isolates the Tor circuit for each account.