I think you're right in that that was the original sell, but it's not how they sell it today.
"Servo’s mission is to provide an independent, modular, embeddable web engine, which allows developers to deliver content and applications using web standards."
So maybe they're just not there yet?
But in my opinion, if you create popular software and no one uses it, it's not popular from its adoption, it's popular for some other reason. I can't point to any one using Servo and figure out how they use it, because I don't know of any companies or products actually using it besides Mozilla.
And I embed web tech, so I've spent a lot of time in that particular field. I just don't see it actually being used outside of Mozilla projects, like it was originally conceptually sold.