You couldn't reasonably claim you did a clean-room implementation of something you had read the source to even though you, too, would not have a verbatim copy of the entire source code in your memory (barring very rare people with exceptional memories).

It's kinda the whole point - you haven't read it so there's no doubt about copying in a clean-room experiment.

A "human style" clean-room copy here would have to be using a model trained on, say, all source code except GCC. Which would still probably work pretty well, IMO, since that's a pretty big universe still.