Is he entitled to royalties for his work?
Legally yes. The law isn't super relevant here though.
Duke open-sourced it, but I don't think it's protected by any particular license like GPL. So, while I am not a lawyer, I think people are free to make closed-source forks of the project, and profit from them. If they want.
That's the opposite of how it works. If you don't give any explicit permission so permission is not given; authorial copyright is the default. But the 4chan tradition is to ignore that kind of law.
Duke original picked this up from a defunct game that was abandoned long ago.
Duke's code has no basis in vbcoder's version. The only potential point of reference is the name, which was first presented anonymously but which Duke ended up moving away from anyway.
Legality doesn't really matter much here anyway though, because aside from GW there's no real possibility of anyone getting the law involved. Duke isn't that kind of person, anyone else who tried would invoke so much ill-will as to throttle any possibility of profit.
IAS is such a blatant attempt at working around the copyright rules I don't think it hold in front of any lawyer.
GW would win any lawsuit anyway, by virtue of being able to send lawyers at all, while this game, being free, doesn't have such a budget. It's better defended by the ability to disappear into the internet with no specific person to sue.
How do I contact the developers?
You don't.