Tone is in the Ears
It's fuzzy, McCartney says it was John who actually quit first, but that he wasn't taken seriously for quite awhile. The others went one like it hadn't happened. It was actually quite a few months later but John, George & Ringo signed up with Allen Klein anyway (in hopes of getting back some of the Apple Money) but Paul refused. Thene Paul said "I guess it really is over" at the point when he refused to sign the Allen Klein contract. It's all in that article. John's reaction was "good, finally everyone has realized I was serious."Again... McCartney was actually the LAST to leave, after the band had already effectively fallen apart – the only difference is, he was the only one to go PUBLIC with it (again, because he used it to help promote his 1st solo album)
These are the facts, and all 4 Beatles have described the same thing over the years – anything else is just purposely argumentative semantics, honestly...
Lennon was mad that Paul got to be the first to say he was leaving, he felt he had that right since he started the band, but Paul beat him to it by announcing his solo album.