Just for reference: metafilter bans self-links (ie links to content that the poster is involved with in any way) outright in posts to the main page. It's extremely unlikely that the poster has anything to do with the product in this post whatsoever. The fact that they're a member of fifteen years standing with only 15 posts to their name makes it even more unlikely.
I wouldn't argue that that this post deserves to rank highly in any search engine, but to downgrade the whole site because of it? Madness: It's a link to an interesting mathematical shape, with a bit on perfectly on topic discussion underneath which contains a few other relevant links. Best of the web? Probably not. Spammy? Hardly.
It is also spammy in that it doesn't nofollow external links on comments. Those links should have been nofollowed like 6 years ago. That they still aren't, is telling about MetaFilter's knowledge on SEO. You know that Google sees your site linking to bad sites, and what do you do? Complain about it? Fire people due to an update 2 years ago? Or do you simply take action to fix it?
Links to spam sites will be flagged & then squashed by the moderators in short order. Well moderated comments by an active user-base shouldn't be nofollow - they contain actual useful information that a search engine (like Google!) ought to find useful, if page-rank matters at all.
It's possible that metafilter might have a problem with links in old posts ending up pointing to domain squatter sites I suppose, but if this is a major problem then it ought to show up in metafilter's google webmaster tools.