I totally don't understand that either. Same with the not!Sheppard/not!Weir kiss in The Long Goodbye. It's not them! You can't use it to support your ship.

I mean, if Cadman had kissed Sheppard in Duet I'd certainly have squeed over the visuals, but - it wouldn't have been a McShep kiss. The McShepness would have been all in the aftermath - and that'd have been fic, not the show.