/who has nothing to do with layers and Blizzard has actually stated that as fact.
Another popular theory is that you can determine your current layer by doing a /who in a capital city, and comparing the results against the /who your friend does. That doesn’t work because /who returns results from the entire realm, not just from your layer, and if the result set is too large it truncates the results before sorting them.
My bets are on Blizz trying to brick gold seller chat channel invites; they’re likely automated with a custom addon that relies on /who.