In startups it's extremely common to have management still write code. Hell I'm CTO and I write a lot of code.
Ah so this was a more narrow example than I initially read it as.
In that case I guess you just keep pressing them to document/make notes. Keep asking questions. Basically take away their “saved time” by dumping the time sink they dumped on you back on them.
I know there can be CTOs who write a lot of code and be good, but I hope to God you're not my CTO because he is a nightmare in this regard right now. We aren't even a startup anymore we have 80 engineers and he pushed 200k lines this week