Years ago I watched a bunch of people stop an apartment building from being built. They did this by employing a legal concern that they didn't actually care about, but that they knew would stop the development in its tracks. It worked.

That was the day I realized that for a lot of people, rules aren't actually rules. They're tools that they can use to stop something they don't like, no matter what the rule is really about.

I think this is a disgusting attitude, but it's unfortunately the way a lot of people operate.

So it might be that Apple has this "no external code" rule to stop things they don't like, and the category of "things Apple doesn't like" doesn't actually include every app that runs external code. It includes a lot of them, but for whatever reason Apple chose not to codify the details. Crummy if true, but I wouldn't be surprised. Every regulator I've ever dealt with leaves themselves an "I know it when I see it" escape hatch that lets them ban whatever they want.

If you read the actual rule the exceptions are relatively well defined. Stuff like pythonista falls into their educational/coding app exception as they define it