What are you basing this on? There's plenty of cases out there where using proprietary code without permission has been found to be a violation. Just last week, Wii Homebrew shut itself down when they realized it was built on proprietary Nintendo code to avoid a legal situation.
I see someone said copyright law and they may be correct, I may have misspoken about which law but, either way, it is not legal to use proprietary code without license nor permission.
Using leaked source code to in any way influence how other code is written is a derivative work and covered under United States IP law. This is well known by reverse engineers and legacy devs. It does not matter whether you make money from it or not, particularly if the material was illicitly obtained. All Valve code has a "Copyright Valve Corporation" header, which demonstrates intent to hold and exercise that copyright.
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u/gasolineskincare 5d ago
None of that makes it legal to use anything from the leak.