I definitely know the difference between a browser and a browser engine; I worked on both WebKit and Servo. There's much more to a browser engine than rendering alone, and rendering isn't even the most complicated part of a browser engine. Outside of rendering and JS there's a steady stream of new DOM APIs, and that's the part that is similar to the "agile user-story kanban".
Yes? It sounds like you're agreeing with me, and disagreeing with your own GP comment. Or maybe I misunderstood the point of your GP comment—it sounded like you were trying to use some sort of metonymy to equate Firefox/browsers in general with Servo, as an implicit counterargument to my GGP post in the sense of "it'd be bad to write a browser in Rust for [reasons you gave]; a rendering engine is part of a browser; a rendering engine thus has the same lifecycle as the browser itself; and thus, a rendering engine is not actually a good example of a good use-case for Rust, despite Mozilla's ambitions. QED."
Clearly, though, you know what a rendering engine is, so I'm not sure what your argument in the GP post was. Was it just a tangential statement (i.e. "rendering engines are well and good, but it'd be silly to write the browser itself in Rust, for [reasons]"), rather than a rebuttal...?
There's certainly a layer of a rendering engine—the part that translates a CSSOM into rendering pragmas—that changes a lot over time. But rendering the resulting pragmas itself doesn't change much, and features requiring new pragmas come about remarkably slowly.