In other words, money that these people don't have. The legal system is not a solution for these kinds of problems, nor it is affirmative defense. Anything that makes the defendant bear the burden of raising and proving that their actions didn't foul any legal requirement, is basically killing any project, even when using your "solution".
To me this still means "there IS no way". You can get sued and convince a judge you didn't do it, sure, but that's not necessarily 100% accurate, and also probably extremely unlikely to happen anyway in most cases. And you'd be surprised how easy it is to fake evidence with no way to prove otherwise. Plus all that still requires going to court.
Generally one has two sets of developers, one doing the RE work, and one doing the new implementation, and the only way you allow them to communicate is through documentation of the reverse engineered implementation. Should this go to court, you can walk each member of each group in to testify, and show off the stacks of documentation produced in the process.
That might convince a judge, sure, although it's still possible to fake the evidence... but I would argue the vast majority of people who claim to have clean-room RE'd something absolutely did not go through anything close to this process.
I don't know anything about the majority of developers, and I think sweeping claims about any group require strong evidence, which seems lacking. But there are plenty of examples of companies that followed such a process, and succeeded in court: https://en.wikipedia.org/wiki/Clean-room_design#Examples
I think when someone fails to produce any evidence of a clean-room process being followed when they claim it's clean-room, is probably a good indicator. Yea you can call that "trust me bro" if you want, I won't be upset.
The Wikipedia article on clean room reverse engineering has all the examples that came to my mind and then some. https://en.wikipedia.org/wiki/Clean-room_design