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...
The idea is not to make users unaware of the mining software running, it's to make them easily configurable to run when the users do not mind it at all. There are API calls in Android (and I suppose on other mobile platforms also) to start an app only when the phone is on charger and not moved for some time. Once you wake up and grab your phone, the app can stop until the next night charging cycle. I doubt anyone would mind that, and if you can do something useful and get some value from using such an app many people would give it a go. Compare that with cost of entry to mine today. No wonder that cryptocoin penetration with general population is as low as it is now.