That's not exclusive to that app. Being able to update the app without going through the App Store was one of the big selling points of React Native a few years back. Apps are not allowed to download native code, however.
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.