Well, isn't that advice also mostly given to founders aiming to create a potential ≈unicorn and not a lifestyle business like this?

Don't get me wrong, what this guy has done is impressive. Growing a company while staying profitable all the way, and being able to support himself is great. I would switch places with him (ex post facto) in a heart beat.

But still, this company will probably never be bought by anyone (besides maybe Stripe), and could very well exist for many years to come with 1-2 employees including the founder.