Apple has a slam dunk case to place a ton of barriers on browser vendors being allowed to JIT code due to the risk to the rest of the platform and the user's data.

Little emulators for NES and Sega Genesis are not going to meet those barriers and Apple will have a pretty easy time excluding them under the security provision.

Dev environments like VS Code will be more interesting. I suspect Apple will choose to build a VM layer to execute complied code inside an even deeper sandbox rather than allowing execution directly inside the iOS layer, thereby severely limiting what a binary complied on device is able to interact with. This would almost certainly be hidden behind an entitlement that Apple will be try to be allowed to gatekeep under the DMA requirement to maintain platform security.