Earlier quoted context omitted.
This comment is only partially true, the realities of managing customers on routers with thousands of routes precludes this careful management at the rock bottom tier-1 transit prices of the lowest common demoninators. The way HE & Cogent operate for large web-hosters at data centers is by trust and prefix-counts. The churn of various web-operators between hosting companies is massive.
Did not Cogent pioneer massive use of multihop BGP to do prefix filtering outside the low powered edge in the nineties?
Starting in 2000, yes.