Yes it seems to go some way beyond what can be achieved with the basic rwx permissions.
I do vaguely recall, mid 1990s, about the time I was moving on to greener pastures, ACLs were appearing in Unix and confusing my simple view of the world. Even so, iirc, ACLs were just glorified permission controls. File attributes like ‘immutable’ seem to have crept in while I wasn’t looking.
I have the impression that in order to accidentally write to an immutable directory, even as root, one would have to first accidentally execute ‘chattr -i’, which does not seem likely.