“Only the UI” is the part of the system that the user actually interacts with; without it, the system is unusable. The free software movement doesn’t distinguish between “stuff that matters” and “stuff that does not matter” - a proprietary app or system that contains free components is still, overall, proprietary. Otherwise, you might as well say Microsoft Edge (Chromium based) or macOS (BSD based) are free software, which they obviously are not.
You are correct - “real Linux” (in scare quotes) is a sarcastic remark about how people do not consider Android to be an actual Linux system (which it is). Judging by the architecture diagram, SailfishOS without the proprietary components just looks like a regular GNU/Linux, which is why I suggested actually free (or freer) alternatives.
Sailfish OS is proprietary. It goes in the trash. If you want “Real Linux™” consider Mobian, postmarketOS, or Ubuntu Touch.
deleted by creator