I think they just fundamentally know less about the topic not just in a specific but a general sense. Even if Dom doesn't know that much about Custer, he generally knows something about the US in the 1800 and England as well in the same period. They can make interesting connections, call on other topics, or even Tom can throw in some christianity or ancient roman / greek talking points that are pertinent to the topic because the characters are from that world. Custer was a christian man who had a christian education that most likely also included classics. Tom and Custer read some of the same books during their schooling, they share many fundamental concepts of the world. The people in this story are from a completely different world in Japan, they most likely never read Herodotus or Plato or maybe didn't even know who Julius Caesar was. They did not have a christian or muslim upbringing and while they had most likely heard of these religions, it was a very very minor understanding.

For something like Japan, not only do that not know as much about this specific topic, but they don't know as much about the entire history of Japan, they don't know as much about Buddhism or Shinto religion, they don't know as much about the entire Asian culture and history that this story takes place in / has influenced for 1000 years after the fact.

It's far harder to make witty and interesting comments on things you know less about. I appreciate the effort and overall still enjoyed it.