Android one phones are near stock version of Android.
Starting Android 8 (or 9) project treble aims to deliver new version of Android without work from the OEM.
Yet phones with Android one and treble don't get update right away, see for example the Nokia roadmap to Android Q:
For example, Nokia 2.2, Nokia 3.2 and Nokia 4.2 all have Android one and start with Android 9 (so they are treble ready) and have to wait until Q1 2020. Why can't they get the GSI directly from Google servers to get update as fast as it's released by Google?