How do we know, or would Feynman know, that they can count while reading and not just that counting and reading are simple enough tasks broken down into discrete steps that our brains can context swap in an unnoticeably short unit of time?

Just trying to read your post while counting at a consistent pace I associate with roughly 1 second per number, it didn't feel like I was reading words but instead scanning them and understanding them after the fact from memory. Usually when i read I hear the words 'in my head' but not while counting at the same time.