does he mean diode as triode with direct heated cathode and cathode plate as a grid?
could be, but that would be a pretty awfull triode i assume, since... well... how should the cathode tube that encapsules the heater regulate the flow of electrons, if those pesky little beasts don't even get out of the cathode tube?
despite the cathode material emitting the most electrons being on the outside of the cathode tube...
so we would have a "grid" that is emitting electrons. "grid" current would definitely flow - which won't be a huge problem since the cathode used as the grid will be much tougher than a thin grid, but that thing wouldn't be controllable in a normal way at all.
And we would have a grid made of solid sheet metal. no electrons from the "cathode" (=heater) could pass to the anode, besides from the few that go outside the cathode tube where the heater enters.
but those wouldn't be very regulated from the potential of the cathode at all I assume, but a little bit of influencing coule be possible.
But - I am intrigued now and would love to try this idea at home. maybe this weekend...