If you don't have an idea, then maybe you should look into buying an existing site that is already earning, and grow it
You can make money if you don't have much interest in the niche. I doubt most people with home loan sites and acne sites built them because they were very interested in the topic. What I think you probably mean is that sites won't grow much if at all when you have no passion for it, so it stunts the earning potential for that site. In which case, that guy wanting to make a huge popular site probably won't be able to accomplish his goal if he has no passion or desire for the site's niche.