is this information correct?

http://www.dpreview.com/news/0210/02100402sensorsizes.asp

somehow i'd always thought that 2/3" or 1/3" was the horizontal width... so 2/3" was roughly 16mm. wow, these sensors are truly tiny!

You are not alone.

My assumption had been that it was the diagonal measurement,
similar to a SLR's 35mm diagonal measurement of the negative.

No no. The 1/3in stuff is *not* a diagonal measurement of the sensor. Its an ancient way of specifying sensor size.

And also the 35mm in SLR film cams is not the diagonal. It's simply the width of the film that passes through a film cam - it's 35mm exactly wide.
The size of a frame of 35mm is 24mm (high) x 36mm (wide).