What I'm understanding, is that the 12th word can't be predicted from the 11 first words. I need to choose a random 12th words, cumpute something, getting the checksum and chose the words that validate my seed ?
If you have 11 words, for finding a valid 12th word, you have to brute-force it. Because for 12th word, you need the checksum and for computing the checksum, you need the first 7 bits of the 12th word as well.
He has complete access to his entropy. I think he's just confusing the terminology here.