The Online Safety Act 2021 (https://www.legislation.gov.au/C2021A00076/latest/text) says:

(1) For the purposes of this Act, social media service means: (a) an electronic service that satisfies the following conditions: (i) the sole or primary purpose of the service is to enable online social interaction between 2 or more end‑users; (ii) the service allows end‑users to link to, or interact with, some or all of the other end‑users; (iii) the service allows end‑users to post material on the service; (iv) such other conditions (if any) as are set out in the legislative rules; or (b) an electronic service specified in the legislative rules; but does not include an exempt service (as defined by subsection (4)). Note: Online social interaction does not include (for example) online business interaction.

Since "business interaction" is mentioned as an example of something that is not "social interaction" one might plausibly claim that "social interaction" should be interpreted quite narrowly, and then one could claim both that the primary purpose of HN is not interaction at all (it's a new aggregator) and that any interaction that does happen to take place in the comments is not "social" interaction but some other kind of interaction.

(It really does say "2 or more" rather than "two or more".)