What's at stake here?
OP has doubt regarding the veracity of an answer to a question that has been closed.
Now OP needs clarification. What could be done?
$\bullet$ Comment! Seriously, leave a comment to the post seeking clarification re OP's concern. The poster, if needed, would surely respond or(and) might edit the post, if necessary.
$\bullet$ If the user is no more in the site, then still leave a comment behind, for there could be others who could notice and respond to that.
$\bullet$ Ask a follow-up question. Explain the concern in a new question re any assertion made in the answer to the previous question. But make sure to explicitly reason as to why it's not a duplicate. That is, it is to be asked with clarity why, at the very first place, the concern is legit and that warrants a different post.
$\bullet$ Also, one thing to be noted is to why the original question was closed: it was marked duplicate. So, one can resort to that question for more info.
Finally, one more I would like to add is that if a post is wrong or disseminating misleading assertions, chances are there would be responses from the community: it's highly likely the errs don't remain overlooked from the vigil eyes, if there is any error at the first place.
TL;DR: Interact with the poster airing the concern primarily by resorting to comment.
Some related posts across our sister sites in the same vein:
How to ask for clarification of an answer?
What should I do if I want more clarifications about an answer posted on an old question?
Clarify an old answer
How to ask for clarification on a very old answer