I think the key point is the bottleneck link is a shared resource. Many TCP flows traversing the link will drive it to a relatively high queue occupancy which

I think the key point is the bottleneck link is a shared resource. Many TCP flows traversing the link will drive it to a relatively high queue occupancy which causes higher delay for all traffic regardless of protocol. Only skimmed the proposal but looks like it isolates traffic using the new protocol by giving it a dedicated buffer, and the explicit congestion notification protocol would then keep the size of this queue much smaller at steady state when the link is saturated.