I believe the 4S did away with distinct models for GSM and CDMA. The cellular chip it uses can handle both, much like the Galaxy Nexus. (Unsure as to why the Nexus supports T-Mobile's AWS bands and the iPhone doesn't though, maybe the antennae.)
I would speculate that the reason carrier unlocked phones do not work with CDMA is related to their agreement with Verizon (and/or Sprint).
On some access points you can disable client to client traffic as a security precaution. If such traffic was disabled, it would probably break the ARP request to see if someone is currently using the IP. (@ 00.0180 seconds in the original article)
I would speculate that the reason carrier unlocked phones do not work with CDMA is related to their agreement with Verizon (and/or Sprint).