Based on this comment:

There are some anomalies in the federation between Lemmy and NodeBB. In the past, I noticed that Lemmy only federated communities/categories where at least one Lemmy user had started a conversation. Yes, I know it seems like magic, but I imagine there is a reason for it…

If what the comment said is accurate, it would be an issue if I want to subscribe to a category where I"m not supposed to create a new topic.

My guess is that I couldn’t see them because the OPs are from Mastodon-like accounts.

This might be the reason. Lemmy has limited support for Mastodon-style Note objects as top-level posts.

just paste the NodeBB URL into Lemmy's search like https://linux-nerds.org/category/8/linux

Not sure if you can pull individual posts, you might have to be subscribed before the post is made in order to see it.

Speaking about the instance I'm in and specifically with respect to !technical-discussion@activitypub.space, I'm subscribed to it. So far, I only see posts that a Lemmy user or a user local to the category/community's instance has authored.