You can watch the entire presentation here.
http://www.gdcvault.com/play/1023446/History-and-Game
Under the tab - There is one man who can explain it all. He talks about Karl Marx & Marxism.
If there was a joke, I didn't get it.
It's not really a joke (although there are jokes), but it's also not what you and others are trying to make it out to be.
Your problem is that you started too late in the presentation and then didn't stick with it long enough. The tab "There is one man who can explain it all..." is in specific reference to the colonization mechanics in Victoria II. If you go back several tabs to where he starts talking about Victoria II, it'll make more sense. (And it'll make even more sense if you go all the way to the beginning. If you're interested in grand strategy game design, it's a good talk.)
For those who don't want to sit through the whole thing, here's how the Marx reference fits in:
The guy giving the talk, Chris King, is a designer of historical grand strategy games. He has worked on some major titles for Paradox, including Crusader Kings II, Europa Universalis IV, Hearts of Iron, Hearts of Iron III, and Victoria II. The talk itself is on the difficulties of using history as the basis for systems you can use to provide engaging gameplay in a grand strategy game. According to King, history is messy: it's full of differing interpretations and it frequently turns on unique one off events, both of which can make it difficult to model.
Marx enters the conversation in relation to Victoria II, which is a semi-sandbox grand strategy game set in the real world during the Victorian era. While players can choose pretty much any country they'd like, the focus is really on the Great Powers, the tensions between them, and their colonial adventures -- especially the scramble for Africa.
The problem, according to King, is that we now know, today, that imperialism was a losing proposition. With very few exceptions, the Great Powers spent more on their colonies than they got back. He says that, in a game that acknowledged that, good players wouldn't colonize Africa, and then the AI would have to be programmed not to colonize Africa so that computer-controlled countries wouldn't be at a disadvantage. As a result, Africa would remain uncolonized and a key source of tension in the game would be eliminated.
The design challenge was creating a game system within which the scramble for Africa and the resulting tensions would make sense.
King goes on to explain that different historians have different opinions about why the scramble for Africa actually occurred. Some argue that it was for prestige, others argue that it was a way to distract people from domestic problems. According to Marx, however, colonial expansion was about securing access to raw materials for a nation's factories and securing access to markets for the goods produced in those factories. That fit nicely with the economic model the designers had constructed, so they were able to build from Marx's interpretation to create a system that would reward players for choosing colonial expansion by strengthening their economies.
That takes us up to "There is one man who can explain it all..."
After that, he highlights the things that make Marx's theory excellent from a game design perspective: it's mechanistic and deterministic so it can easily be converted into a logical game system. He
does talk about Marx in glowing terms, which seems to be a sort of a joke in the sense that it's an ironic lead in to his next point, which is that choosing one historian's interpretation doesn't mean endorsing their views or peddling their philosophies. He actually takes a light-hearted jab at Gene Roddenberry for doing that. Far from making an argument for Marxism, he goes on to say about his use of Marx:
"There's no need to start the revolution, because historians don't always agree. The cynical would probably remark that that's how they make their money, but I never said that. So it always pays to shop around. Look at the various historical interpretations and find the one that's going to kind of dovetail nicely into your game, what game experience you're trying to offer people, and then look for the historian that kind of will tie these two things together."
In context, he's actually saying the exact opposite of what people are taking him to mean.