The main technique to do so is "parallel construction": e.g. you reverse engineer a binary or look at source code, find out a special case not implemented in Wine, then construct a test case that tests the special case and behaves differently in Windows and Wine and claim that you found the test case experimentally, from a personal project that didn't work in Wine or from open source code you find that triggers it (rather than from RE or source code reading); you then implement a patch in the way you find most optimal without influence from how the code you reversed or read did it.

This is in fact good for Wine since it results in both having high quality contributions and having no additional liability.

So Wine's policy effect is not to stop such activity, but to make the Wine project not liable for it; thus, any leaks of Microsoft's source code are in fact good for the project since they enable this activity and the Nintendo situation here is similar.