I'm the Technical Lead of Chessable and the exact person who integrated SF on Chessable.
There are no license issues here because web app is not the same work as a Universal Chess Interface (UCI) chess engine.
Thanks for trying our SF integration!
The engine and rest of the app is not considered a standalone work, which is what GPL is about. It's like Chessbase shipping their GUI and Fat Fritz 2. Did you ask Chessbase for source code of their GUI? The SF engine in JS is not an executable linking to the app. It's actually you who chooses a browser and run the SF. It does that by running web workers like Chessbase on a standard UCI protocol. Stockfish is not a library or derived work, it's an independent piece of code that your own browser runs it.
PS: Any views here are not my employer's and purely mine.