Professional opinion from a very dedicated Destiny 2 player is that the game is not an MMO, but...
The game itself is more of an RPG nowadays but it's gone through so many phases that I'll disagree and say I can't exactly blame someone for trying to argue it's an MMO. A lot of core systems are ripped straight out of the MMO book or had been in the past, as the game directors have been pretty big WoW fans. Terminology too. Probably the overlap in fans makes a lot of those fans consider it one, too.
Main point here is I agree with Pippa there but she has a very traditional definition of an MMO that doesn't exist in the same capacity, and might not in the future. It's unfortunate but years from now the common idea of an MMO might apply to... Fortnite, of all things, as an example.