The 40CU PS5 die is rumoured to be around 310-320mm^2. It is unlikely those additional 16-20 CUs would take 90-100mm^2 (assuming a fifth gddr6 controler) in the XSX. It's still a guessing game in the end.
Ya it's all educated guesses, some more than others, but if you follow his logic and reasoning, based on AMD's past, present, and future forecasted sales, along with the more reasonable console spec leaks, it's surprising that adds up so close to AMD's wafer allocation for next year.
It could be completely different than what he thinks, but to be that close without making any wild assumptions, well.