I would suggest clarifying the documentation that the test returns false if either file doesn't exist. And perhaps add a CAVEAT that other shells have different semantics in case one of the files is missing.
I can't make sense out of the ksh bevaviour. I could make sense out of "true if file2 doesn't exist" for both -nt and -ot (you want to assert the condition that file1 is up-to-date/outdated wrt. time stamp file2; if file2 is missing, no condition needs to be fulfilled). Apart from the equal-time condition (is that relevant any more now that micro timestamps are used?) zsh's -nt/-ot seem to be designed to behave exactly inversely to each other even in the corner cases?