Good catch! Apple would likely close off the most common avenues to provide such an app. There are nevertheless many ways a developer could make it available. Off the top of my head, one could distribute it as a webapp, or use an exploit to install, or provide as an Xcode project to compile and install directly.

It also is possible someone will force Apple to permit third parties to run app stores.

I should point out that while I chose Apple in my comment, there will be other brands with fewer restrictions.