Since there are probably millions of these, why not recycle into cheap servers given the benefits of their low power use (and the possible ability to draw power to charge during cheap periods and run off battery during expensive ones).
Becoming a node on a botnet to save $5 is not very attractive.
That said, RPi still has one or two binary blob driver holdouts as well, but I agree there is a world of difference between them in this respect.
The slides are light on details, but if I were doing this, the phones would not be running Android in any case, so while security is an important consideration, you can't just say "but security!" and toss out the baby and the bath water.
On the other hand, if enough devices of the same type become available for this purpose it might make sense for a business to do this at scale.
The only reason that's true is because groups have catered to the device and have ready-to-go instructions. Were the same effort put into making Samsung S2/S3/S4 etc phones easily converted, I'm not sure the time difference would be all that much at all.
This could then be used as a basis for phone recycling programs: manufacturers could run the collection (and e.g. hand out gift cards for its own new products to encourage the users to turn them in while also profiting from it), and then sell the collected phones in bulk to datacenters.