Agreed: sub Romans were Christians and their shields may well have sported Christian iconography. It's just that the cult of Mary flourished long after the supposed date of the battle of Guinnion where "...Arthur carried the image of the Holy Mary, the everlasting Virgin, on his [?]..." If there's any historical veracity to the 'Campaigns of Arthur' section of Historia Brittonum, then the line I've quoted is definitely a later interpolation.