Yea, either / or. It's a multiparadigm forum system.
I will admit, a few days ago, I found the answer to 3 threads, which itself (a simple quote from the Discourse Meta forum,) did not deserve it's own topic thread. So I went ahead and posted near the same small post in all 3 to hopefully answer their open questions and close out their discussions.
So to me, it is more important to answer (or give a solution to) any open thread, than to add clutter by forcing members to open new threads on the same issue or subject.
I feel the guideline that reads "Don’t cross-post the same thing in multiple topics.", is a bit miswritten.
It should say "Don’t cross-post the same question in multiple topics."
(With a sub-rule: First search the forum for existing answers, and then start a new topic if your questions has not been answered.)
This rule is aimed at users who post technical problems, or modeling questions in multiple semi-related existing threads, which wind up soliciting multiple answers, or repeated advice. Basically it makes more work for staff and sages.
Obviously there is a unspoken exception, and that is it's okay to post the same short answer (or advice pointing at the common answer elsewhere in forum or help center,) ... as users tend to ask the same (or similar) questions because the great majority of new forum members do not take the time to search the forum, and even less take the time to read the guidelines.