This is not correct.

Firefox still uses Gecko for its HTML engine. Quantum was a project to incorporate some learnings from Servo, and other larger performance projects, into Firefox components, including Gecko.

Just an aside, but Servo was never intended to replace Gecko, and was only intended to be a R&D project for improving some Firefox components. This was due to the long-tail of web compatibility that would be required to make Servo a suitable replacement for Gecko.