I hadn’t realised that a topic here was closed and planned to reply by using the “Quote” option.
It defaulted to “Post as new topic” which was confusing. There was another option hidden in the top left to “Reply to existing topic by…” (or words to that effect). I used that, and accidentally posted to a completely different topic.
It would be simpler if there were some warning about the topic of the quoted post being closed.
(At the same time, it is useful to be able to quote from a closed topic. I think the past that wasn’t possible.)