Self promotion within limits is fine for HN. See, for example, this valued contributor who posts links useful content they've created. Most of these appear to have been upvoted, not downvoted.

https://hn.algolia.com/?query=author:minimaxir%20minimaxir&s...

Rather in the context of the "link to a paid product" or a startup they own without disclosing it, as the OP was implying. I should have clarified.

For comments linking to my own blog/projects, I very intentionally a) disclose I've written it and b) try to only post it when it's extremely relevant to the topic at hand.

Sure, but Parent poster is linking to accounts who are also making the same disclosure. "Here's this product I ahve". And while I didn't look through the entire posting history of the linked accounts, the mentions don't seem too frequent.