Kuma's mind is going to be erased when his surgery ends
Kuma's surgery takes 2 years, Bonney's surgery takes 6 months
Kuma wrote the letter after Bonney's surgery (6 months), not after his surgery (2 years)
Once Kuma's surgery was complete, he became the mindless cyborg that we saw as a slave in Marie Geoise and that we saw rampaging it