I have been riding mountain bikes since 1985 (Yes I know a lot of you weren't born yet) and riding E-Mountainbikes since 2015. I have found that I am a lot fitter because of a few reasons. I try to explain this to people when they inquire about my E-bike as if they think it is just for old people or people who have injury problems.
I ride multiple times more than I would on a regular bike and I don't feel the recovery lag I would feel on a regular bike. I am able to go longer distances with a lot more vertical than I would ever want to ride on a regular bike. I am able to push hard for the entire ride without stopping and keeping my average heart rate up continually to a very high level for most of the ride. Even on the downhills with the extra weight of the bike I am able to keep my heart rate where I need it to be. I don't feel it is productive to max redline on steep accents where your heart is ready to bust out of your chest. I could be wrong on this and I am sure I will hear plenty of rebuttals on that particular point.
I guess everyone is different in how they define keeping fit but for me, the E-bike has far surpassed my vision of fitness compared to a regular bike as I move towards my 70s and I hope I can keep my fitness level where it needs to be. Plus when you enter in the fun factor comparison the mental benefits that contribute to overall health and well-being are over the top for me.
I hope I didn't offend anyone's opinions, this is just my honest straightforward opinion and feeling like a 30 something.