I agree with you when speaking of wooden rods (bamboo, greenheart), but not when speaking of tubuler fiberglass, boron, or graphite. Since spline is defined as the slot or groove that joins two parts together, it seems to me that when speaking of the high side of a rod blank, spline is the proper term. This because when the prepeg is wrapped on the mandrel (or the bamboo strips glued up into the blank), the place where they join produces a spline, and that is where the high side of a tubuler rod blank is found.
However, when speaking of bamboo, not only is there a spline for each strip, there is a spine, the stiffest strip in the rod section. And with greenheart, there is only a spine and not a spline.
At any rate, since the two of them are used to refer to the same thing by most fisherpersons, IMHO, it matters little which one someone uses.