That's ridiculous. There's a screenshot at the top of the page; it's clearly on Mac OS X, and it isn't as if it claims to be a universal app for the whole planet. Personally I think if the page began with a giant "ONLY ON MAC OS X" disclaimer that would look more like he only cared about that.
It was just an oversight in the Hacker News post title which has now been corrected.
No, it isn't ridiculous. I've seen many projects available for multiple OS's that choose to advertise themselves with a Mac screenshot (or a Windows screenshot, or a Linux screenshot) at the top. The screenshot by itself says it runs on Macs, but doesn't establish that it's Mac-only; you need to read all the way to the bottom to make sure of that. It's disdainful of a non-Mac user; that may be accidental, but I've seen this more often on Mac product sites than on Win/Linux product sites.
Your "giant ONLY ON MAC OS X disclaimer" is a strawman; what would be normal and courteous is simply mentioning that it's for OS X somewhere at the top. There are many, many product sites that do it successfully (e.g. http://www.hogbaysoftware.com/products/writeroom).
The post title on HN is a separate issue that's actually less of a problem. A random link to a product on HN doesn't carry an implicit assumption that it'll run on my OS. I don't mind clicking on it and discovering it doesn't (and besides, the author can't control the text of a link that leads to their site). But when I need to scroll down five pages on the site to ascertain I can't actually use it, that's irksome.
It was just an oversight in the Hacker News post title which has now been corrected.