Actually, there is a problem in the README. There was an issue where Linux did not deal well with composite devices that have both relative and absolute properties on the same node, and handled the relative parts only. Thus, absolute mouse didn’t work at all. A patch exists that makes things work, but in the end, we spun absolute mouse into its own node, and the issue under Linux became moot.
In other words, Linux handled relative mouse fine. It handled absolute mouse fine too. But both on the same HID node made it (libinput, and it’s X11 driver in particular) very confused. But since we have these on separate nodes now, the issue does not apply.
The README should be updated.
For what its worth, here’s the Bugzilla ticket: