idkdan
Well, in order to make them look "celtic" for greater audience (that we expect will play Bannerlord), we have to reach for some slight anachrony.
Having in mind that the fan favourite faction Vlandia is almost anachronistic in its entierty, it is certainly permitted for Battania, in order to improve upon the visuals, worse to none.
The average gamer has just a vague image of a "celtic look" (in big part shaped by tye movie industry), and doesent mind if that armor is from 6th or 9th century etc.
Far from that the average gamer is stupid, on the contrary, they (all of us) have noticed the Battanian underwhelming and non-defined appearance, but just a very small part of them are that nitpicky about the smallest details that don't change the bigger picture whatsoever.
These are also celt, now a disclaimer I'm not for implementing ALL of the equipment in the images (especially the sword on the last one), but to follow that general look, with the ones you've shown.
Three words : Tartan, tartan, and more tartan !