Well, most people do not consider "mountain biking" to be anything done on a mountain bike. If I sit on a trainer in my living room on my mountain bike, I am not mountain biking.
Mountain biking was an example of a hobby that is very difficult to pursue unless you have child care arrangements, whether it is day care, a baby sitter, or only while a partner has free time on a weekend.
I don't even mountain bike. I'm very confused how you're reading so much into the example, and simultaneously not by saying riding your bike in the city is what I was getting at.