Market validation - especially for online products and services - is indeed incredibly important, but not many people like to write about it because it is actually very drab and a tiny bit audacious.
His market validation - you are seeing it. It is a submission to Hacker News.
Ok ok, that is a given, but the author alludes to other various ways he gets feedback and validation for his products. He also talks about how well his products were received on Product Hunt and you can also follow him on Twitter.
My final advice for seeing how online services validate their ideas - run the product's domain through a backlink checker. It lets you see what other websites link to their product and what avenues they pursued for outreach. Each of those submissions exposes the product to potential customers and gives valuable feedback to the creator.