A “hidden” topic is one that isn’t listed on topic lists, i.e. it isn’t “discoverable” in the normal fashion. You can tell a hidden topic by the eye with a line through it symbol.
Actually there is a way to automatically make posts from the WP Discourse plugin “hidden” 
You can use the “Publish as Unlisted Topics” setting.
Keep in mind both what I said up top, and what it says next to that setting. This will mean that topics published from Discourse to Wordpress do not appear on the topic lists of your forum. Comments will work in the normal fashion. If you have the sync comment data webhook enabled the topic will no longer be hidden after the first comment. That feature wasn’t exactly designed for this purpose. See further
If you want to just add a X-Robots-Tag: noindex header to an embedded topic (without bothering about this hidden business), you’ll need to either request that as a new feature of Discourse itself or add it via a plugin.