I'm not sure I agree, they mention HAProxy and Foo over UDP so they are leveraging existing open source technologies. Custom additions to suit ones particular use case isn't necessarily the same as NIH syndrome.
Joe from GitHub here, we'll talk about it later posts but GLB is based on a number of open source projects including, haproxy, iptables, FoU and pf_ring.
Many existing open source solutions are optimized for short lived HTTP requests and don't address the long running connection issue (like a large git clone). We wanted something better for our use case.
I'm currently working with GitHub Support on dealing with zip downloads of a 5GB repo failing after 2-3 minutes, with curl error "transfer closed with outstanding read data remaining".
Sure about the long running connection issue being solved? :-)