Yes, elliptical chainrings are great IMHO, especially if you train for elliptical continual exertion. Added 2km to my speed.
No it did not.
It is hard to say if it is implausible without the time unit. 2km/day? I believe it. 2km/s? Seems unlikely.
Ha! Yes you're right, units matter. For me it was 2 km/hour, on rides for an hour or so typically above 20 km/hour, on a hybrid bike, on long streets in Marin. The ellipticals worked especially well, for me, in combination with bar ends.
It's not that far above what some people were claiming when they were new. There were people claiming 1mph improvement. I was... skeptical. Placebo effect? Who knows.
I don't think I ever got around to trying one. I was the youngest regular member of our club. Power output was not my primary problem.