Boot, and then what? Are there open drivers to the iPhone peripherals? Last I checked, there weren't: only having access to the CPU is not entirely satisfactory.
That's the point of the article: being capable of running code on main CPU is but the first step, drivers and firmware for all the other parts are far more problematic.
Hm, isn't it theoretically possible to use the checkm8 exploit to boot your own code? Is the problem just that writing your own OS for the iPhone is difficult, or is there anything actually blocking you once you have code execution in the boot ROM?