I don't know why everyone is so sure that confirmed standby works here.

There are three separate questions here:

#1, and the most important, does confirmed standby apply to first class fares? Nothing we've seen indicates that it does, so, if it does, what class needs to be open? For Y, it's H. For F, it's ...
#2, can you do confirmed standby for a flight after your flight's departure time? Yes.
and, #3, can you do confirmed standby after having missed your flight. I'm not so sure that the answer to this is yes, and have yet to hear anecdotal evidence to that effect.