There's nothing wrong with empty comments saying positive things like congratulations. See https://news.ycombinator.com/newswelcome.html, which was written well over 10 years ago.

It's not as if stories like this come up very often.

This whole thread is a little bonkers. A dude posts what seems to be essentially a commercial for his company, extremely short and light on details and it gets 500+ upvotes in a couple of hours. But yeah maybe i have just become to cynical and my bullshit metre is off - and if that's the case i apologise and congratulate these dudes.

For what it's worth, I tend to upvote most "Show HN" threads, because I want to encourage new things. I've never seen a "Thank HN" post before, and thought that it was worth upvoting because it's nice to see success stories, especially when it's due in large part (according to the OP) on the advice given by this community when the product was in its infancy. I thought that was pretty cool.