Tennis requires a certain proficiency to have fun with. Beginners tend to have trouble getting the ball reliably across the net onto the other player. This proficiency takes time to build. Thus, unless one makes a big up-front time investment, tennis is not particularly good exercise. Up-front time investments are expensive.
Also one cannot tennis alone. Anything one must practise with a partner is more expensive due to scheduling requirements.
> "expensive" to mean something other than wealth, i.e., time.
I don't think they did say that. They just said wealthy people have more freedom on schedule that non wealthy people.
> They just said wealthy people have more freedom on schedule that non wealthy people.
I'm not sure that's true though, unless by "wealthy" you mean trust fund kids. But there are millions of tennis players of various levels of income. A lot of salaried workers in upper income brackets work more than the usual 40 hour week, have less free time.
I'm guessing these engineers weren't playing a lot of tennis: https://www.folklore.org/90_Hours_A_Week_And_Loving_It.html
The OP was talking about monetary wealth. Here you're redefining "expensive" to mean something other than wealth, i.e., time.
Also, the whole point of the submitted article is that the investment of time into exercise is totally worth it.
Yes, there's a learning curve to tennis, as with any sport. You could just go jogging/running by yourself, but the advantage of sports, including tennis, is that they're usually a more fun and less boring form of exercise than jogging/running by yourself. If exercise is fun, then you're more likely to stick to it rather than skipping it.