That’s true enough for a single cycle of a single transistor, not so much a billion of them doing a billion operations per second. Which is why they are calling this “vacuum-like.”
They generally aren't. Vacuum as-in the medium may be technically superior, but vacuum tubes as practically implemented weren't just displaced because solid state was easier to manufacture, but also because solid state quickly outperformed valves in many areas. (There are areas where valves held out for much longer and are still used today in some cases, e.g. transmitter output stages).
That being said a great thing about valves is that they're very hard to kill. They can take huge momentary overloads (a bit like magnetics, but with much shorter time constants), which would literally detonate similarly specced solid state output stages. And because they can run much hotter than any known solid state tech they can have huge energy densities and can be cooled very efficiently (boiling water cooling): https://upload.wikimedia.org/wikipedia/commons/8/80/Rs2041_s... (Siemens RS2041V, 600 kW output tube, diameter of the copper anode dissipating about 240 kW is 239 mm, was still made in 1999)
Doesn't matter, as explained in the second paragraph of the article: the air gap (35 nm) is shorter than the mean free path [1] in air, so the electrons are effectively moving in a vacuum.
> Is this basically the return of the thermionic valve after fifty years in the wilderness?
Yes. The paper's abstract starts out saying just that: "Scattering-free transport in vacuum tubes has always been superior to solid-state transistors. It is the advanced fabrication with mass production capability at low cost which drove solid-state nanoelectronics. Here, we combine the best of vacuum tubes with advanced nanofabrication technology." [2]
[1] https://en.wikipedia.org/wiki/Mean_free_path
[2] https://pubs.acs.org/doi/10.1021/acs.nanolett.8b02849