It's an unfortunate situation and I agree with you that it was a valid question that merited an answer to a specific problem the user was having with his code.
Perhaps the code in that question can be edited to remove the unnecessary details that provide the complete solution (including deletion of his test class since the issue was with his base code). Edited to add, that's exactly what I have done. I hope others agree with this solution. The user's issue is solved and the offending static variable is still visible. Only the framework of his solution remains.
With that having been said, complete or nearly complete solutions are frequently provided as answers in the Developer Forums; primarily to "buggy" or poorly written modules. However, with a "Super Badge", I'd really hate to see solutions to those posted anywhere and most definitely not on SF.SE.
I wanted to add that we had a very lengthy discussion in Responding to Trailhead Challenge Questions that led to the wording you've quoted in your OP after the first question or two was posted here asking for help with a trailhead challenge. It's a fine line to draw between excluding questions related to Trailhead and letting people know that we're not going provide complete solutions as answers.
What was unique about this particular question is that it was essentially already a nearly complete solution that just needed a minor tweak in order to "fix" it. With most questions that are asked, with a few exceptions, it's usually a case of where the user hasn't looked at the documentation that's referenced by the module. Perhaps this a "heads up" to us that we need to be watching for questions that are asked in general and editing them after resolution to remove irrelevant code that provides too much information.
I can think of a few modules that were very poorly written initially or were very buggy when released where that wasn't the case. That said, SF.SE really isn't the place to be asking about those when that's the situation. There's the Trailhead Group on the Success Community and of course the Developer Forum's Trailhead Section where the "right people" (like Jeff Douglass) are there watching to help address those issues.
Unless you've already gone through a particular module you wouldn't be aware these issues exist or know what the workaround for them is. What you've quoted was essentially a compromise that allowed Trailhead questions to be asked, but prevented complete solutions to challenges from being posted in answers. Perhaps what's currently written should be edited to include that "the best place to ask questions about Trailhead" is either of the two locations I've referred to above for precisely the reasons I've just cited?