The only unusual limitation is that you're not allowed to say "built with Sciter" without getting permission from the maintenaners. You can still use it for commercial projects though.
That's just the sdk shim. The underlying engine looks to be closed. (My question was unrelated to commercial use - that's only one potential benefit of FOSS.)
https://github.com/c-smile/sciter-js-sdk/blob/main/LICENSE
The only unusual limitation is that you're not allowed to say "built with Sciter" without getting permission from the maintenaners. You can still use it for commercial projects though.