Even if Galaxy is running under Wine:

• It’s a package manager. It handles downloading files and updates, installation and patching, and verification.
• It integrates various GOG services, like cloud storage for save files.
• It can set environment variables and pass arguments to launched games.

Besides, a Linux-native port doesn’t need to package anything. It can simply mark Wine/Proton and various compatibility solutions as dependencies.

I’d assume that would be part of it, yeah. But that may indeed be a faulty assumption. Anyway, achievements don’t work without a client even if they’re native Linux titles.