“From that period” doesn’t really apply in Arthurian mythology, since there are a number of historic figures from different Dark Age periods that may or may not have been the basis of Arthur. Plus Arthurian myth is traditionally portrayed with lots of anachronisms (plate armour for instance) so really things like accurate ethnicity cease to be an issue.

In the case of Dev Patel (an actor I love to bits) and this film, he is absolutely remarkable in the role and fits it like a glove. It doesn’t come off as stunt casting or diversity virtue signalling in any way whatsoever.