While I don't disagree that most of HR is cargo cult bullshit, the hotmail signal is real. Having worked in customer support hotmail users really are as dumb as a box of hammers [0].
I'm being sarcastic. The joke is that it would be the worst possible signal to send out.
AOL was an internet provider that mostly died out in the mid-2000s. Back then, "true nerds" scoffed at it -- it was for grandmas and total noobs.
So if you got an AOL email address (which you still can actually do, turns out), it not only is a relic of ancient technology, but it's a signal that you're a technology noob in like the 90s.
I just am amused at the idea of sending a resume to a trendy tech company with an AOL email address. They'd either laugh or flip out.
0. The usual exceptions for anything human.