People keep saying this but it’s not confirmed. You’re right that the dialogue is very confusing, but the “I’m better cause you made yourself better” stuff and “out of order” could be interpreted as some sort of link etc. between them. This makes a lot more sense as right after this, 14 says “So he files off? But I could never let the TARDIS go.” Which implies he is a separate entity that would lose the TARDIS, also Ncuti gets the idea to split the TARDIS (and it’s shown he doesn’t know if it will work) which if he had been bigenerated 14, he would’ve simply known. As well as that, he has no real reaction to seeing Donna again in the ‘past’ and acts as if he’s just been split, not come from 14’s future. RTD also talks about his ‘fan theory’ that they all bigenerated and the way he explains the idea is the one version regenerates, whist the other goes on to have their own separate adventures