The thing is that, as mentioned by Alvaro and Andres on this thread, we have no guarantee that the different relation locks compared have a monotone hierarchy and we may finish by taking a lock that does not behave as you would like to. We are now lucky enough that ALTER TABLE only uses ShareUpdateExclusiveLock, ShareRowExclusiveLock and AccessExclusiveLock that actually have a hierarchy so this is not a problem yet. However it may become a problem if we add in the future more lock modes and that are used by ALTER TABLE.
Please provide the link to the discussion of this. I don't see a problem here right now that can't be solved by saying