From glancing at the schem (assuming that is the correct schem) it looks like the PS-ON net controls a photo transistor that ultimately controls the "Fast Fault" pin of the switching controller which appears to be maybe an onsemi NCP1396 for which the datasheet reads:
"Fast Fault: Fast shut--down pin. Upon release, a clean startup sequence occurs. Can be used for skip cycle purposes."
So it kinda sounds like if you ground the PSON# pin momentarily, it will stop driving within ~55ns.
Reading the datasheet regarding "Fast Fault" says:
Skip cycle possibility: The absence of soft--start on
the NCP1396A fast fault input offers an easy way to
implement skip cycle when power saving features are
necessary. A simple resistive connection from the
feedback pin to the fast fault input, and skip can be
implemented.
So if they are skipping cycles by watching the feedback network it certainly sounds like a legit thing to do.
Although it sounds like there are 2 versions of the chip that behave differently. And I'm not 100% that the schem and chip are correct. I just did some hasty googling.