The original algorand paper also talked about forking and didn’t have the soft vote / certification vote steps so frankly can and should be, ignored. It’s definitely been a source of consternation over the years that it has’t been replaced/updated and people have instead had to just point to the specs repo.
Fabrice talking about 20% is likely around that being the approximate threshold that can halt the chain (which is certainly an important threshold !). For full control, proposing a block and guaranteeing it was accepted, you’d need 74+% of all online stake (and across multiple accounts you controlled). With the comparison to Bitcoin - the 74+% threshold makes more sense (IMO at least) for the NC of Algorand.