Ah, the variant of classic 'code documents itself'. Which is true unless you come to a dark place where stuff stops working randomly and you have no clue why. Or you interact with other systems which are just blackboxes for you.

Its fairly trivial to write on few lines piece of (bad) code that is really hard to grok for anybody else and even for you after few months / years.

What good documentation does, and it can take just few lines here and there, is explain why things are supposed to happen or not, expectations, corner cases etc. Haven't heard any good argument against that in past 20 years and doing this myself diligently even on code that is purely for me. Always thanking myself when debugging something old.