Of course the most successful people you mentor are the ones who are only doing what you are guiding them to do and aren't distracted by life, it doesn't mean they are better.
This sounds like propaganda given by the CEO to have more effective employees. There is no doubt that focus can help you improve and get there faster, but at what cost and with what life goals?
Side hustles can be fun, profitable and life changing. We've got at least 3 that has allowed my partner to reduce her work hours to 4 days a week and generates half of her salary. We can buy fun stuff and business expense it and if we continue to improve we can gain more control and freedom.
Original post was about having hobbies and not feeling guilty about not turning them into side businesses. It would be better if our culture was set as a default to say "that's awesome, it's so good you could sell it if you were so inclined, but feel no pressure to do so, I just want you to know it's that good", instead of saying "you should make a store/service to sell your hobby" and make people feel bad, even though it's just a compliment.