Does one expect someone less experienced here to know to type 600 in the export, Bitmap Size box?

No, indeed, that's the problem.

No, an image on the web can be expressed only in pixels because we assume we know the resolution of our display.

We don't need to know the resolution, but we need to have an idea of the kind of ratios that exist between screen pixels and browser pixels. It's that ratio that counts. It used to be 1:1 but now browser:screen can be as much as 1:2.5 (or more for all I know). It's that ratio that matters.

Xara does spit out retina images (at 2x resolution, I think), so there is some understanding, and currently that will always give a good image, though some of the Samsung phones have very high pixel densities.

DPI really doesn't matter. It's browser:screen pixel ratios that determine the optimum bitmap size.