You are reading it wrong, modern dual/quad core ARM chips would mine with comparable mining power to ordinary botnet node, considering that they will mine while phone is absolutely idle (night) they may even be on par with desktop computers which have to do something else while they mine.
Botnet mining also needs to remain stealthy and fly under the radar.
Memory intensive PoWs are particularly troublesome because you cannot easily throttle RAM usage like you can CPU usage. Users are very likely to notice when you accidentally send their machine into swap-hell...