The text you quote is fairly clear to me: "Focus on the content, not the person". Any statement that amounts to "You are X" when X is a negative quality is rarely going to be a good thing to say here. But if you say something like "Doing this would make you X" or "It would be X to proceed without..." instead focuses on the behaviour.
Imagine an OP who posts a question because she didn't run a company-standard background check on someone she thought was very trustworthy and it comes back to bite her after she hired them. Posts that say "You're so naive, you shouldn't have done that." are not constructive and would likely get removed. Same for any comment saying "How naive." which would get a rude/abusive flag from me.
But it's different if an answer says "It was naive to just trust this person, you should have still followed procedure. In future do A and B to...". That just describes the situation and the assessment is on the actions taken by the OP, not their character. Even something like "you were too naive here" is still fine, provided the answer focuses on giving advice to learn from the experience or do better next time. Any post that instead focuses on pointing out some perceived failure of the OP or their character for no constructive reason is really nothing more than a veiled insult.
A useful test is to see whether you would describe what you're saying as "calling them X" rather than "describing behaviour as X". The former is not appropriate here.