You can make the same argument about Minecraft since servers download you texture packs, data packs or skins. I don't think that Apple should stop these apps, that's their game model.
Roblox games are not comparable to texture packs or skins. They are complete games with assets and executable code, or at least the fancy ones are. Maybe data packs are more similar but it seems like Minecraft for iOS does not support them.
Seems like they use JavaScript and this might qualify for the HTML5/JavaScript exception that Apple has. Also it seems like they are a lot less powerful than Roblox scripts, and there isn't an in-game store allowing you to purchase them without using Apple's in-app purchase flow.
Yup! Soft currency (Minecoins) travel with you with your Microsoft account and can be used on what ever device you sign into (including iOS). You can also purchase more Minecoins in the game its self each platform.