There are several code editors for iOS https://duckduckgo.com/?q=ios+code+editor and Apple has even published an own app for students to learn coding called Swift Playgrounds https://www.apple.com/swift/playgrounds/ - it isn't a full-blown code editor, but it clearly allows you to write and run code.
And that is ignoring cloud/browser-based code editors. That execution model (write code local, run in the cloud) makes sense with mobile devices which have limited power supply. I personally don't want to spin up several docker containers on my iPhone ;)
But other people might? Especially if it not a phone but a 12.5 inch iPad that is supposed to be more powerful than my 5-year-old dev laptop (and has more RAM too).
> It is allowed to write code on iOS
Great, so you can pen&paper your code like in the 1960s. In the 21st century, writing code includes being able to run the compiler/interpreter on your terminal to debug as well.
That being said OSX running on a higher specced ipad pro CPU could be really awesome but I don't think that is what will happen.