I'm not familiar with that particular app but if that happens it's not fine and it seems to get a pass from Apple. The apps are not allowed to download executable code. You have to bundle it all in the app.
There is a special entitlement to allow some Apple apps and frameworks to do this — Apple’s JavaScriptCore can do JIT, which requires executing data pages. Similarly, Swift Playgrounds can compile to native code and execute that code
So, for example, you could ship a C compiler on iOS and it would compile perfectly valid binaries, but you would never be able to execute those binaries natively without code-signing them and deploying them in an Apple sanctioned way, which you cannot do on-device (except with the above mentioned exceptions)
Only for Safari or if you use a WKWebView, otherwise if you use JavascriptCore yourself on iOS, JIT is disabled and quite slow. On macOS it’s enabled.