In your quote, they didn't want to create a full browser, but they did want to create a full engine. And it was designed to be "productizable." I think it supports what I'm saying.
Servo was the last big plan from Mozilla to turn things around. Since then, we've just gotten Pocket, random bundled add-ons, etc. It seems like they are mostly just content to slide into irrelevance. And I am very sad about that. I do honestly wish Servo had succeeded, although I thought it was pretty obvious it wasn't going to.
Also, while I'm airing grievances, I think it's very likely that having a few big chunks of Firefox/Gecko be written in Rust is going to be a maintenance burden that will slow down development even more. Now that Mozilla has laid off the Rust and Servo people, what are they going to do?