The instance I'm logged into doesn't forward my user agent, IP address, or CSS/script support (or other fingerprinting techniques) to the other instance. Everything I do in a community hosted on another instance is forwarded through my instance server as a middleman, and I never directly connect to the other instance server.

The admins of an instance (or reddit) might be able to analyze server logs of different users on their own instance to be able to determine those things, but can't apply that analysis to accounts from other instances, whose interaction with the server doesn't actually include a login or any direct connections to the server they administer. All they have to go on is the ActivityPub logs, which won't include that fingerprinting information.