> Actually even GPL is not a protection here, with sources given away they become almost a manual how to re-implement it which is far simpler than making it from scratch (oh and yes, people are doing that without copy pasting).

That's quite a gamble, since it falls foul of the Clean Room Implementation criteria

https://en.m.wikipedia.org/wiki/Clean_room_design

Big companies (faang) won't touch that approach with a 10 meters pole, but obviously I'm not surprised that smaller actors might have a more cavalier attitude to copyright law.