Interesting, maybe the /e/ updater is less strict 
This is definitely the case for vanilla FPOS, there has been a discussion before about this: Can't install Dec 2021 update
They should probably be checking the hashes as described in Step 5 from Life of an A/B update in the Android documentation…
In any case as I understand it (and I’m by no means an expert here), there is no such thing as two usable slots on a phone with Android Verified Boot. The second slot is only used during OTA updates, but once the unused slot (with the update) has been successfully booted, Rollback Protection should make it impossible to boot from the old one.