Moreover, you can't just have hoardes of flying vehicles randomly over populated areas (and if this was at all practical, every company and its dog company would be trying to do the same thing), so you'd need fairly strict regulations on where they actually fly, and a system for managing them (e.g. limiting the number of flights by all companies). They'd probably need to do things like limit the paths that could be followed, maybe above public roads (which would reduce the delivery radius). Whatever happens, the (necessary, because it involves public safety and contention for a limited common resource) regulation and government control is going to slow the process down, probably a lot more than Amazon's timeline suggests...
Even if they could grind through all the problems, get government support, and setup a system to do final delivery of very small purchases by air... is the investment required to do all this worth it to be able to deliver a vanishingly small proportion of packages more quickly?
What this really looks like is an unintentional leak of Amazon's next April 1st gag...