Hmmmm. I'm thinking that leaving the bamboo in its natural shape, i.e., concave/convex might have been the expedient way to build a bow in Howard's day. To flatten the bamboo would have required additional milling and probably reduces the overall strength of the bamboo a little. Though they probably had the capability to do it in Howard's day, it would have added time and cost to the construction. On the other hand, I have no idea how much hand work might have been involved in working with the natural shape of the bamboo. Using slats/laminations close to the finish size without milling would have been the easiest, I suspect. Looking at the substantial thickness of the laminations in some of those early bows, it seems that's what was done at least some of the time.