This is not quite true. If you write your own app you can install it on your iOS device. If someone put an iOS app on, say, Github, you could easily install it. No need for the App Store or Apple's review process. An app does not need to be signed unless you want to submit it to the App Store. Also, you can install iOS without using the App Store if you have a developer account.
I'm not disagreeing with you that iOS is far less open than Android.