Firefox Quantum isn't a rewrite of Gecko. At one point, Servo was going to be a rewrite of Gecko, but that petered out. They pivoted to a strategy where some parts of Servo would be integrated into Gecko, and other components of Gecko would be rewritten in Rust. The most significant of these was Stylo, Servo's CSS style (but not rendering) subsystem. However, despite the rebranding, the core of the engine and the overall architecture is still Gecko.

The thrust of Spolsky's argument is that you can't stop delivering just for the sake of a rewrite. If a rewrite happens, it has to happen in the background while your main product keeps shipping. Mozilla spent years letting the pieces that would become Quantum incubate in Servo, all the while still shipping Firefox as normal.

its rendering engine hasn't been completely re-written though. They've been re-writing parts of it piece by piece.