Far as I can tell, "lowest highest point" is a comparison of the extremes, with no tricks.
As far as "mean elevation" goes, I think you can substitute the word "average" for "mean". So *in general* Delaware would have lower points than florida, but as for absolute highest points, Florida has the lowest. I think we're saying the same thing, but I've been awake too long, so I might be interpreting badly. My apologies if so.
I haven't looked up figures on mean elevation, but if you come across Delaware vs. Florida figures I would be interested just for fun :)
Having just learned that the question came from a trivia card in the first place, I have nothing useful to add to that end :)
Feeling inspired, I'm going to try to write a differential equation that describes Delaware's topography. We will not be able to solve for a unique solution to Delaware's topography, but we can see how it behaves.
Sleep would be good right about now.