I thought this was a really innovative way to cut miter splines. It's not perfect as the interior cut-out of the spline will be circular such that the spine won't seat squarely on its back end. But there's still probably enough surface area to add strength on the top and bottom so long as you have a snugly fitting spline.
I hate building jigs. Only have so much shop time and building jigs isn't how I want to spend that time (although I do appreciate the ingenuity that goes into an elegantly designed and executed jig). So I'm always intrigued whenever I see a way to do something well without a jig.