I agree, writing code is writing. I added a parenthetical above that hopefully clears that up. I guess my overall point is that I can't confidently say I've even had an actual idea, until I prove it to myself by voicing it, or writing it in code or prose, or drawing a diagram, or whatever. Thinking is hard, but it feels good to have thunk, so the mind is incentivized to give itself the illusion of having done so, if it can.
"thinking in code" means "render their ideas in code", like you render your ideas in English.