The Gameplay video or trailer field on the Edit project page is a different story.
Pity. I wonder if that is some kind of policy, as they provide the trailer in the thumbnail whithout visiting the game page. So as not to break compability or the site, by having arbitrary embeds. Or even malicious ones.
Because from a technical viewpoint, it should work out of the box, if they would accept an embed there.
I might be a minority (or of the majority, who knows), but I never ever activated any trailer in the hover pop ups. So for me there is little difference, if a trailer is in the description.