The article seems to be fairly clear about this: it is Google focusing on Google phones (so unlocking the bootloader should not be an issue) and they did mention that the kernel would have to be replaced (albeit for other reasons).

I would think the main factor against such clusters is cost. Even if the four year old phones are free, they have to be dismantled, tested, and supporting hardware/software has to be developed. All of that would have to be done on an ongoing basis. While Google may have the volume to be able to build uniform clusters with a given generation of hardware, generations are measured in months. Using four year old hardware also trims four years off the expected life expectancy of the components, and that is comparing like to like (not consumer grade hardware to server grade hardware). I've got to wonder how all of that extra work affects the carbon-footprint they are trying to reduce. It would probably be more effective to increase the use life of the phone as a phone.

All of that is fine for a research project or, on smaller scales, hobby projects. It would be extraordinarily difficult to make it commercially viable.