I mean it's a cute story and an idiom that is pretty much indelible from the language, but Kay was putting it in terms like it's a fact about frogs' biology when it is not. He has a background in biology as well as CS, so I am indeed surprised he didn't check this.
Appears to be inconclusive: "None of these modern rebuttals – Melton, Zug, or Hutchison's – attempts to replicate an extremely slow-heating experiment as cited by Goltz or Scripture: only Hutchison did a thermal trial at over five times Goltz's slow rate and over nine times Scripture's 0.002 °C per second temperature rise." [1]
More interesting is perhaps the question of how you get the frog to sit still -- or maybe change the experiment so there's a grid over the top of the pan -- and try to see if the frog gets more and more "desperate" (flight from danger as opposed to "just a frog jumping around") as the water slowly heats?