I think the reasoning is about having alt accounts for different purposes. He intention is to map one human to one account and have all of their thoughts from that one account, instead of one human having one account to discuss scraping on, and a different account to discuss crypto on.
I'm pretty confident it's not that.
HN's prime directive is "anything that gratifies one's intellectual curiosity": <https://news.ycombinator.com/newsguidelines.html> and many, many, many dang comments.
I'm pretty sure that the specific gripe is posting excessively (not even necessarily exclusively) on a single topic or theme. See <https://news.ycombinator.com/item?id=19392902> for a more detailed comment from dang.
Occasional alts are explicitly permitted, though not to engage in abuse (e.g., mutual admiration societies, sock-puppetry). See: <https://news.ycombinator.com/item?id=9963551> <https://news.ycombinator.com/item?id=9823379> (both against sock puppetry) and <https://news.ycombinator.com/item?id=9122086> and <https://news.ycombinator.com/item?id=7504621> (on where throwaways are/aren't permitted).
Where HN does favour persistent accounts the stated claim is to foster community, rather than for nefarious tracking purposes: <https://news.ycombinator.com/item?id=18082346> and <https://news.ycombinator.com/newsguidelines.html>. From that last:
Throwaway accounts are ok for sensitive information, but please don't create accounts routinely. HN is a community—users should have an identity that others can relate to.