Looks like the gsi (general system image) that vendors need to boot to demonstrate treble compatability - is mostly tested for exactly just booting/starting. So any phone that supports treble will boot gsi/boot an image derived from gsi - but not all phones will work with gsi - for a meaningful definition of "work as a full android phone".
In other words treble/gsi seems to be a bit of a half-measure - and if you "port" à given rom to treble/gsi - it still will only work on some subset of treble phones :'(
This is what I gleaned from some searching, anyway:
There's also annoying variations in GSI's - you need separate images not just for 32-bit vs. 64-bit but for A-only vs. A+B boot support, and sometimes for different "base AOSP" releases - GSI's for devices that shipped with Aosp versions 8.x, 9.x, 10.x are not always guaranteed to work consistently.
In other words treble/gsi seems to be a bit of a half-measure - and if you "port" à given rom to treble/gsi - it still will only work on some subset of treble phones :'(
This is what I gleaned from some searching, anyway:
https://www.xda-developers.com/unofficial-lineageos-17-gsi-r...
(after first finding this, and being a bit confused):
"Changelog 16 - Smart Styles, Treble is trouble and Omfg Oreo"
https://www.lineageos.org/Changelog-16/