You're not missing anything. The big/big+ method confirms the shortest safe length for the chain. But the chain can be longer, and when using RD on gear ranges less than their full capacity, there's a decent amount of room for a longer chain.

Also, in low gear, your chain isn't on bib/big. Shift to that and you'll see the cage rotate to nearer horizontal. Then convince your self that it can go yet more horizontal by pulling the lower loop forward (keep the crank from turning) until it's straight.