People would say that but most firewalls block _incoming_ port 80 connections so that logic never held much weight.

That isn't really relevant to the problem they were working around. The problem is that many firewalls block outbound ports other than port 80, 443, and some other very common ports.

Put another way, if both sides block 80 incoming, their only hope is fancy NAT-punching techniques.

But those NAT-punching tricks are useless if they are using a port that is completely blocked on the outbound side.

Many networks like in hotels and airplanes only allow out on port 80 or 443.