Hmm, yeah that looks buggy to me - seems the string is not optional. If you add anything to the string argument - even a space - it works. (an empty string doesn’t work, tried it)
arrPoints = Rhino.GetPoints(True, False, "Select leader points")
If IsArray(arrPoints) Then
Rhino.AddLeader arrPoints, , " "
Edit - the Python/rhinoscriptsyntax version works with two points and without a string entry - but, I discovered a new fun wrinkle: placement in the Front and Right viewports with only two points:
As two points determine a line, there is no specific plane, and looks like it then defaults to World. Methinks if a plane cannot be determined default should be the active CPlane, not World XY…