A EULA might but I’m not positive that is the case. That said, I would expect a business of any size to cover their ass and not bother without a compelling reason.
That said, look at the steps involved.
Person A had the original software in binary form, runs it through a decompiler. The decompiler is not going to reproduce the code precisely, but it will be enough for documentation purposes. They document the functionality in a separate document describing what is happening when the software is run.
Person B receives this document and writes their own code according to what it says. They need not have ever even used the original software or signed a EULA, and then Person A or C or whoever can go on to test compatibility. It is a lot of effort, but the end result is code where no one involved violates any laws. It’s interoperable, and because the code is original, Person B or the Project he works for can claim copyright and license it out on their own terms, in the case of ReactOS, it is an open source license.
Re: Oracle vs Google, this case does make the whole project potentially suspect these days, but if APIs are not copyrightable, then ReactOS should be firmly on the right side of the law. If APIs are copyrightable, then we can expect the Wine project to be taken down alongside ReactOS. More reason to hope Google prevails entirely, regardless of our own personal opinions of them.