I think that's the rub, in my theoretical scenario, Apple is not blocking the distribution or sale of iOS applications through third-party means, they'd enforce their existing restrictions on and power over building iOS applications in the first place. Developers would absolutely still be able to distribute unsigned applications - end user iOS devices would just be unable to install them.
It sounds ridiculous to me, and as I wrote earlier, it would be a clear violation of the spirit of the DMA, but I don't see any reason why this scenario would not be technically possible for Apple to pull off.