It does add a little strength to the joint. Not enough to be worthwhile for ordinary cabinetmaking but he has a good reason to do this. (To be clear, this joint is less strong than using Dominos the usual way, at 45 degrees to both parts)
He carves the parts, sometimes carving around the corner.
Using the finger jointed tenons at ninety degrees to one another allows the tenons to stay in the middle of the boards keeping them well below the finished surface of the part compared to the usual tenon reinforced miter joint which uses a tenon at 45
degrees to the part surface so the end of the tenons are just below the surface. He probably exposed one while carving...