I'm pretty sure barbenheimer was astroturfed to shit as well. The memes and content people were making didn't feel anywhere near as natural as the Doom x Animal Crossing stuff did. That trend felt legitimately homemade whereas anything similar that's come after it has felt incredibly forced.
But honestly I didn't even know about barbenheimer until after we watched Oppenheimer and already planned to watch Barbie.
I think the only 'phenomenon' was that there were two good movies out at the same time that weren't super hero movies