@Askorti
That's what I was getting at. Only explanation was that Ryouma's martial arts are OP
@SunSun
I suppose that would be the case then. I didn't think of that. The shield must have been very badly damaged, and the wood glue was shoddy. I can't find references for the effectiveness of medieval wood glue, but modern wood glue creates a stronger bond than a single piece of solid wood to itself. If the shield planks were stuck together with wood glue as good as modern wood glue, then the wooden part might as well have been a single piece of solid wood.
Still, the metal plate, even if it were relatively thin, shouldn't be easy to cut. Just for reference, historical plate armor is usually 1.5mm thick. Since the wielders could just use strengthening magic, weight is of no issue, and they could afford the plate to be thicker.