If my geometry is right, that suggests to me that the very edge of the blade won't be a straight line, but instead a segment of a circle. A rather large circle (radius equal to the long wooden support), but still slightly curved.
Interestingly, I'm struggling to thing of a scenario where that would present a problem - if the chisel is doing a through cut, then it doesn't matter; for fine paring by hand, it's not going to impact; and if one was cutting a blind mortice - then a little gap at the bottom isn't a problem (indeed, often recommended, to allow for glue squeeze out). For something like half blind dovetail (which I don't think are a common joint in the Japanese tradition), then any slight curve in the base would be eliminated when paring to the line.
And for a plane blade - that's pretty much an ideal shape.
The more I think about it, the more there seems to be to commend it; however odd it may seem. Anyone think of a scenario where the gentle curve would present a problem?