Edit Splines > Deform Landscape to "Only Selected" broken?

Is this a bug or am I not understanding something?

See my screenshot below. I have ONE spline control point selected in my landscape. I move it up, and I want to deform the landscape to adjust for just my “Only Selected” control point (thus, fill in the gap beneath it). However, when I select this button, nothing happens! What am I not understanding here?