TBC, I'm not talking about the article, which I've barely read but looks rather misguided as it seems to be talking about LLMs having to be GPLed because of training data, which is not something that would ever happen.

It has never been the case that including GPL code in your software automatically makes your software GPL or even requires you to make it GPL. If you do get sued because you are distributing GPL code in a way that colloquially "violates the GPL" (technically, rather, in way that is not covered by the GPL or by fair use or any other licence, so it violates copyright) you might choose to GPL your code as a way of coming into compliance, but doing so is neither the only way to achieve compliance (you can instead remove the GPL code, and companies with significant investments in their proprietary code typically do that), nor a remedy for the harm done by your copyright violation to date, which you will typically have to remedy financially, via damages or a settlement.

As for legally testing, you seem to be to wanting a court to explicitly adjudicate against something so obviously wrong that in well over 20 years of FSF enforcement (edit: actually around 40 years) no company has been daft enough to try and argue it in court.

It might help if you try and delineate exactly what sort of case you'd accept as proof of "enforceability" of "virality". I think it would have to be something like a company embedding GPL code in proprietary code and then trying to argue in court that doing so is explicitly permitted by the GPL, and sticking to their guns all the way to a verdict against them. I'm not sure whether that argument would be considered frivolous enough to get the lawyers involved censured, but I certainly doubt a judge would be impressed.

If it helps make it any clearer, if in defending against a case like this your lawyer were to try and argue that the GPL is invalid and somehow just void, you should fire them immediately because they're trying to do the legal equivalent of shooting their own feet off. The GPL is what allows distribution of code, and allowing things is all it can do, because it is a license (not a contract). It can't forbid anything, and removing it from the equation can only decrease the set of things you are allowed to do with the copyrighted code.