That's perhaps the best-produced Fleetwood Mac song, which is saying something. Three singers, each triple-tracked, same technique used for Bohemian Rhapsody.
I doubt they used externally-sidechained compression on that recording. It just wasn't a common practice in the all-analog era when it was recorded. More likely, it's straightforward aggressive broadband compression from something like a 670 or 1176. And plenty of organic distortion from a classic amp (currently uses Orange, but back in the day it was an Ampeg).
However, that even-ness is a hallmark of a very good player who can modify his plucking intensity in real time on a note-by-note basis. An easy way to fake it in the digital era is using a dynamic equalizer.