I think most of what he says in the video is true.
Though for me it all in the eyes. This might sound insanely ridiculous but I've noticed the pupils of a lot of gay guys seem a bit bigger than straight guys.
Genetics? Maybe, but it's something I've observed. There's also speech patterns, intonation (in some cases very it's subtle but if you listen closely enough you can hear it, almost like a faint accent) and there's also body language.
A lot of men have very bad gaydar indeed.