More "you're allowed to look at the source" than "open source" though.
> License
This code is released without license. I hope that you'll read the source to answer any questions you have and take that knowledge to your own projects, rather than forking and augmenting Tinkersynth itself.
(Of course you're more than welcome to try modifying the code for your own curiosity! Just please don't publish it, or try to monetize it).
As for “not really open-source”, you’re right. I’m clearer in the FAQ on tinkersynth.com; it’s not “OSS” in the conventional sense. I just didn’t know how else to phrase it in the limited HN title.