If you try to do some kind of dupe-detection, someone can use a lightweight LLM to make superficial changes until it's considered a different project.
Finally, the meatspace status quo is that it is totally acceptable to pay someone to find security bugs in someone else's open-source software, such as the Linux kernel.
Even if you don't, a lot of source code can be legitimately copied thanks to the GPL/MIT/BSD/etc. I'm allowed to take all of zlib and integrate it into my own project if I so chose.
Your private fork doesn't meet the conditions described.