Copyright can also apply to code rewritten in another programming language, if the overall structure and functionality is identical. Although tbh, that is a bit of a grey area, and in previous rulings there is some precedence that if the “translation” involves significant human effort then it can be considered to be sufficiently transformative. But promting an LLM to “rewrite this in Go” very likely doesn’t pass that relatively high bar.
The study you linked is interesting (and likely underestimates the issue*), but it also shows that it is possible to automate detection of copyright infringing code in LLM output, and even just 2% is more than enough to open a case for malicious litigation. Large companies can write that off as the cost of doing business, but for small individual open-source developers this is a significant risk.
‘*’ Because human programmers usually reuse existing licensed code as libraries, while LLMs are prone to rewrite them, and as explained above a rewrite is at an elevated risk of being considered copyright infringing.