Note that there isn’t an absolute correlation between high heart rate and high oxygen uptake (which is the adaptive purpose of vo2 workouts). In a sustained hard effort, your slow twitch muscles fatigue, and you start relying on less efficient muscle fibres to maintain the same power, which puts a greater demand on the heart. You’ll see this effect even in longer steady sweetspot intervals.
So even if you look at “time in heart rate zone” and the Zwift race has you in your top zone for longer, I wouldn’t necessarily take that to mean you were operating at a high % of your vo2 max for longer.
If you have WKO, Golden Cheetah, or Intervals.icu (and some accurate data fed into them), I’d be tempted to use something like W’ / W’bal to see how far you were burning through your above-threshold capacity over the course of the workout.
My general hunch would be: Zwift races are great “give you some intensity to maintain your current fitness” sessions, but not so great “drive aerobic adaptations that will increase fitness” sessions.