Copyright law only has teeth when it’s owned by corporations,

100%. It is funny how any individual can be sued for copying a handful, of pretty much anything copyrighted, yet these AI companies copy literally thousands upon thousands of copyrighted materials.

cleanroom reimplementing technique does still seem to create a derivative product

Will likely have to wait for a case to go to trial, but in theory at least, it is possible these clean room implementations may pass a legal challenge. The youtube video I was watching about this topic had phoenix technologies as an example (for those of us old enough to remember what that company was). In their case it was even more so; they took a commercial piece of software and reverse engineered. If that is possible, then doing similar to an open source software may be considered legal, but again we probably won't know until something like this comes to courts. Different countries may also treat this differently so we will have to wait and see.

The “good” news is this is pretty rare these days.

Sadly yes. But even those that don't make money, or much money, must feel demoralized when someone steals their code.