Also, I think RPF filtering is a little less common than you're implying that it is.
People are always advocating egress filtering because "if only everybody would do it ...", but it's a classic tragedy of the commons. Egress filtering doesn't meaningfully help the network doing it and it may cause ugly problems with asymmetric routes and the like, so the number of networks that don't do it is large enough to be meaningful. And if you get close enough to the core of the internet it's basically impossible anyway because there is no practical way to keep track of which address ranges a particular interface should legitimately be sending traffic from when the list encompasses half the address ranges on the internet and can change at any time.
The upshot being it's not at all difficult for an attacker to get hold of a connection that doesn't do egress filtering, and that isn't ever going to change.
Their method doesn't even attempt to keep the connection going.
Anyone with network experience would see the bruteforcing method as elementary and common knowledge really, it's nothing new.
In short, amateur hour.