Interesting idea, but I suspect if you tried it you'd quickly run into practical limitations. I know people who've tried running build farms with "development board" hardware have found that the stress of 24x7 100% compute hits kernel bugs or overheating issues that 'normal use' doesn't, and I suspect mobile phones would be similar. The pace of phone evolution means you'd have an ongoing effort to get new kinds of devices into your 'cluster' (figuring out how to root them, identifying how to network them, rigging up some kind of custom rack-mounting hardware, etc). And there'd be a bigger turnover of 'replace expired device' than with fewer newer designed-for-server nodes.
The paper also suggests running a hypervisor on these, but I suspect you'd find that firmware/boot rom locks you out of EL2 (hypervisor mode) on most hardware.