You are correct that the pubDate is ignored. The topic’s timestamp will be set to when the topic was created on Discourse, not when the RSS post was initially published.
I’m not sure if there is any way this could be prevented. The feed item’s URL (the value of its link attribute) is used to determine if the topic already exists on Discourse.
The easiest work around for this is to unlist the topic instead of deleting it. That way it won’t be re-created.
The plugin could be told only to get the last n items from the feed based on pubDate, so if the link of an old item changes it would be ignored.
This is was I originally thought Maximum number of posts to embed was for (hence some of my initial confusion).
This is what I did in the end.
Putting it all in perspective, I can see that the RSS plugin works all right, and after the initial few items the pubDate and topic creation date will be the same, and usually it won’t be a problem to create a new topic for every item in the feed.