Do you have a link to the rule about asking questions with only a "single correct answer"? Out of curiosity, I tried digging around and couldn't find references to that kind of rule.
"List question" is meta jargon on stackoverflow that longtime users will understand. It falls under these "open ended" questions, hence why OP's question was closed as "primarily opinion-based" - IIRC it used to have its own close reason but got rolled into that when others were needed.
The question mentioned by the original OP was "My last question (just now) asked about potential maintainability issues involved with a certain approach to CSS layout." which doesn't appear to be very open-ended, at least from my interpretation.
I'll be honest, I mainly knew what the answer to your question would be, but I tried to put myself in the shoes of a newer user trying to understand what questions a person should ask on SO and was curious of maybe things were different.
And the answer appears to still be some form "you won't know until it's closed".
If your goal is to come with any reason to label a question a "list question", then I don't think it would be too hard to find a way to label any potential question as a "list question" and have it closed.
A different interpretation would be that the question is about whether there are maintainability issues with a certain implementation of CSS. Which the question being potentially answered with a "Yes/No" where an obviously better answer might provide an example or two of issues that could arise.
The actual rule here is about question where every answer is equally valid, so asking for pure opinions (https://stackoverflow.com/help/dont-ask). Questions that have multiple potential answers are valid questions in general.
There are questions that are somewhere in between, something like "which approach is better here?". Those tend not to go well on SO even though they theoretically could be on-topic. This is a case where the community is sometimes overzealous, but it's extremely hard to consistently apply these rules to complex edge cases. And many questions of this kind do not fit to the rules and are closed correctly. These questions usually require quite a bit of familiarity with SO to formulate them in a way that works on the site.
https://meta.stackexchange.com/help/dont-ask
"List question" is meta jargon on stackoverflow that longtime users will understand. It falls under these "open ended" questions, hence why OP's question was closed as "primarily opinion-based" - IIRC it used to have its own close reason but got rolled into that when others were needed.
Here's a meta question specifically about list questions that says basically the same thing: https://meta.stackexchange.com/questions/139618/are-list-que...