According to Froboy's wonderful breakdown of what does what in Demigod, I learned that in addition to Impulse/Stardock and Gas Powered Games, a third party's network package was used called RakNet.
After reading recently that GPG wouldn't be using Stardock anymore, I became concerned. Frogboy has since clarified to say that this was a misunderstanding, and that GPG has a great relationship with Stardock and that GPG would not be using the RakNet for their multiplayer solutions.
So I spent some time on RakNet's website, to figure out what exactly their package was, and what else used it. Besides Demigod, it seems to be a long list of small indie projects. This lead me to the conclusion that while Demigod's issues stem from all of the things Frogboy has talked about in compatability between parts, the problem was compounded because RakNet is not a big company, and is not experienced at being a large-scale solution.
Then I stumbled on this: the only thread mentioning Demigod in their forums.
It seems that such a big-time use of RakNet was completely missed by their forum admin. He says that "It is nice of Demigod to credit RakNet, although it wasn't used in that game. Stardock is using NAT punchthrough in the games they host. We're working with them now to adjust their configuration to improve the success rate." Huh?
And continues in a later post saying:
"2. Demigod didn't use RakNet. "The bad design decision was made in December of 2008 when it was decided to have the network library hand off sockets to Demigod proper. In most games, the connection between players is handled purely by one source. For instance, in Supreme Commander, GPGNet handled the entire connection."
I am so lost.