"How Important Is Historical Accuracy In A Historical Video Game?"

That's entirely up the the creator of the video game. If the creator of a video game puts importance on historical accuracy, then historical accuracy is pretty important.

"As Dave discovers, criticism of Kingdom Come: Deliverance emerged because the game fails to include any non-white characters. While the game's designers defended their decisions as merely accurately reflecting the time period in medieval Bohemia, the decision was perceived as being alienating to some players."

And there's nothing wrong with that. If you're put off and cannot find enjoyment in a game because it doesn't include x race or x number of races, well, that's actually rather sad, petty, and pretty racist of you. You are not the targeted demographic, no one has to cater to you.