They almost certainly don't want to deal with races involving incoming data and a request to decrease storage volume.
That's almost right. Most modern filesystems are okay with resizing upwards, but very unprepared for downwards (even when lots of bytes are not yet touched!). So it's really just the "restriction" from GCE that comes from "normal file systems don't like this".
Disclaimer: I work on Google Cloud, but I'm not a filesystem expert.
Honestly it is trivial to do a full dump and reload a new instance with a smaller disk. Like 5 minutes and 3 clicks easy.