This smacks of a special case processing for some specific site. I suspect there's some wildly popular service which presents a page with just a password input for returning users who haven't logged in for a while but still have their cookies/sessions active; the page is oddly coded and the standard approach doesn't work so it needs this workaround; and the page is valuable enough for Apple users that this was deemed reasonable.
The first comment threads are all echoing derision of Safari as a janky browser but I feel that this is something that could be dissected 20 years later in a Raymond Chen-like blog [1] with how they had to painstakingly add a workaround in a newer version somewhere deep inside to make some questionable piece of software not crash or something.
