Really I think you are just being nitpicky.

Destiny incorporates many MMO elements into it's design, and just like other genres there is more to it than face value.

Genres are stupid, and arguing over them is pointless. You want a good example as to why look at RPGs and games like GTA.

Hell the genre action/adventure is a fucking joke, and covers everything under the sun. I have watched people argue Tetris is not a puzzle game, and is actually an action/adventure game.

My view on MMO would be MMO comes down to design not player count. Remember it is 'massively' not 'massive', one is an adjective and one is a noun. It is about feeling massive, not literal player counts.