Disagree, a follow does not imply association, i can follow someone or something for a wide variety of reasons, all the way up to it being an accident and therefor completely meaningless (because someone was included in a followpack for example). Two mortal enemies might 'follow' eachother, but that in no way means they associate with eachother, on the contrary.

Next mutual follows are asychronous, whereas establishing a 'friend'ship is a synchronous occurance.

Lastly the point of musig2 is efficiency compressing it into a single event, instead of two.

So the difference is that a 'friend'ship is one deliberate concious action between two (or more, how cool is that, gotta love the cryptomagic) in a moment of time.