This isn't validating demand or even a business model, it's just "Get to the front page of HN". And if 15,863 people visited your site and only 13 registered your conversion rate is 0.08% not 1.8%.
I calculate conversion rate based on the number of people that hit the course page. That's industry standard. My goal was to test how many people that read about my course signed up. The blog post drives traffic to the home page, but I don't count blog readers if they don't hit the home page because that wouldn't be testing the right metric.
Conversion rates vary by business. A media company tracks the number of readers that click an ad. A mobile app tracks the number of visitors to the home page and download the app. This was the metric I picked to see if people were interested.
I think all you have shown is HN is a source of extremely ineffective marketing for that idea. Your "what's next" seems premature too because HN can't get you "10x more traffic" and even a 10x better conversion will still be barely 1 registration per thousand visitors.
Tomorrow your old front page posts will send you almost no traffic, or ever again, so where is the business model let alone the validation? Still in the testing part, imho. $900 up but no closer to sustainable.
I think you could convince 13 people amongst a group of 15,863 to buy just about anything and that doesn't really prove demand either.
What would be sustainable is finding something that still delivers registrations/customers/money next week when your HN posts are unpopular and it's all tapped out. Somewhere that can actually deliver on the 10x more traffic you aspire to have next and thus worth optimizing your sales funnels for.