My interpretation is this: There are two versions of TWRP 3.5.0_9.
TWRP 3.5.0_9 for ‘9-Pie’ and earlier devices they will be built from the android-9.0 branch.
TWRP 3.5.0_9 for ‘10-Q’ they will be created from the android-10.0 branch.
Warning: The Samsung Galaxy S9 is no longer maintained. is not joyful ews. So is S9+ 'star2lte’. It is likely that the Galaxy S9 / S9+ will sooner or later suffer the same fate as the S7 / S7 Edge … :((
But - Galaxy S9 / S9+ are Project Treble supported device and with /e/ GSI ROM 10 and custom 11 both devices can be used as degoogled smartphones for quite some time.