A better comparison would be between (a plastic base + steel stiffener plate) and (a CF base). They’d have the same strength, but CF is much lighter than steel… hence switching to CF would result in less weight.
This sentence is quite vague…
What brendan may be saying is that people who go through the trouble of switching bases, also go through the trouble of customizing the foam that goes into the saddle. Most people start with foam from a previous seat and shave it down to a shape that “fits” their body better than the stock seat. In the process of shaving, they reduce the overall weight of the saddle.
I’ve found that for short rides, especially ones where you aren’t in the saddle much (such as tech. muni and trials) the foam isn’t even necessary. I use a 20" inner tube on top of my CF base and have been quite happy with it.
yes people go to CF bases because they snap the standard ones, and want something stronger and stiffer. The weight reduction is rarely enough for people to bother making the switch purely for this reason.