The strength of a lugged joint comes from it's surface area. Having the butted mitre maximizes that area and also allows for an internal fillet.

But I wouldn't be surprised if that is overkill. The pierced BB is well triangulated I guess so that it works. I built a frame with a one-piece headtube and it seems fine as well. In that case the sockets are pretty thick (heavier) since they are structural.