Oooh careful talking about fitness here some people get quite offended when you talk about ebikes and fitness

. Yes, even on an eMTB forum, we have those whose egos are bound up with their years of training.
I love the topic myself - eMTB for health & fitness. Probably because I can't bear the monotony of walking/running/swimming/road cycling.
You can still push the pedals as hard as you want in Turbo. I have had Turbo rides with heart rates hitting the ceiling. So the mode of the bike or it's level of support doesn't even matter - it's how hard you decide to (or automatically) work those pedals.
How hard, how long, how frequently - for any fitness activities these are the variables that will produce a certain result.
If you ride with a low effort for copious hours many times a week you get the equivalent of aerobic base training like a road cyclist would pre-season. This is exceptionally healthy in the real sense of "healthy". The heart literally gets bigger, the mitochrondria in your cells work better, resting heart rate will lower.
Conversely you can kill yourself with extremely high heart rates all the time (but for less time) and not really make much progress. Somewhere in between the two is probably optimal for most of us.
The thing is fitness is specific. If you aren't riding an amish bike, why would you need that particular combination of aerobic & anaerobic fitness?
Personally I think as an older guy that a combination lots of time on an ebike having fun (with low/moderate heart rate) and some basic resistance training is all the fitness we need for longevity, health span and general day to day fitness. And I KNOW it's healthier than grinding very high heart rates for hours per week dragging an Enduro bike up hills. That stuff led to chronic exertion headaches for me - because it's not healthy. Ask a doc, or any fitness expert and they will agree.
Another thing to think about - I have plenty of rides where my heart rate is maxing on the descent, I assume from muscling the bike around, but there is likely an adrenaline factor too I suspect. I came to realise that the limiting factor for speed on trail most of the time is my ability to move the bike quickly. This is why downhillers and enduro riders focus on strength work. So don't think that descending isn't exercise. The more you let go of the brakes, the bigger the workout.