I'm not a coach, but those are just version of Chronic Training Load (Fitness), Training Stress Balance (Form), and Acute Training Load (Fatigue). If you look those up there is a ton of literature on them.

Strava's calculation may be some marginal variation on those, but it sure seems to correlate really well with the "real thing." And neither is anything new. The original concept has been around for what has to be decades now - used in products like WKO - and Strava has had it for at least 3 years or so.