Exactly. Even though you could specify what happens mechanically on any given system, the results are kind of uncontrollable on most real computers.

Whether things should be UB or implementation defined requires a good value judgement. Per se there is not a real need of UB, (assuming that the behaviour of a machine can be specified), but on the other hand there isn't much value in specifying a situation where all control is lost to the point where implementation details leak (runtime structures are overwritten etc.).

Since there is (I assume?) an expectation for "implementation-defined" to have an actual definition, pedantically an implementation leak would require all implementation details to be codified, and thus set in stone.