Ah, I see. I am sorry, I forgot you did mention the samurai example. (Btw, Ghosts of Tsushima looks neat. I have no time, but it looks really good.)
Personally, I wouldn't care unless the story hinged on you all being from the same place. You would be playing a samurai. But I do not fault the DM for wanting the flavor to be static. In his mind, he is serving a solid chicken noodle soup. You want to add an Asian ingredient. He says it doesn't go together. You show him a chef (and many Chinese restaurants) that do offer their own flavor of chicken noodle. He can (and I do not think you will like this answer) say, no we are sticking with our original chicken noodle soup recipe.
The fact that you clearly demonstrated he is wrong. The fact that evidence is 100% on your side. The fact that it seems like he is being a stickler when he could bend. It is not how the DM is envisioning things. The simple fact is the DM might be envisioning other problems with your samurai and not able to articulate them - like them being a freakshow or your honor bound morality getting in the way or that other players will suddenly want to do the same and the campaign will be thwarted or even something as simple as a language barrier. There are many reasons. I do not know as I am not your DM. But just because you prove something doesn't mean the DM must change their mind.
That said, he should listen, talk with you, work with you, and bend (imho). But, I would ask this:
Did you know you were playing an Egyptian style game? Did he ask you to make an Egyptian style character? Were the parameters set prior to you making your character?
I am not accusing with the questions above, I am legitimately curious, as I do not remember if you said that information. Thanks.