The article isn't about high performers. It is about world-changing genius. If you had a competent tutor, you would progress much faster and much further than you have by yourself.

Having a tutor is superior to simply having access to all of the information in the world. The tutor acts as a guide to that information, they structure the presentation of that information to you based on your individual needs which means you can quickly speed through parts that you pick up easily while spending enough time and having a ready source of answers and guidance on problems that you have difficulty with allowing you to master them faster. Sure, someone could reach the same level of understanding with nothing but the internet and motivation but they will arrive there much, much later than someone who is actively tutored from a young age. Think of it as like acceleration to some maximum speed, the one that reaches that speed earlier will be able to go much further during the time allotted. You could see that allotted time as the lifespan of the person but age also affects performance and could result in a drop off in ability over time.

An intelligent person who is privately tutored can be performing at a college level or above when they are in their early teens. They can be at a doctoral level when they hit their twenties and then spend the entirety of their most productive years making use of their education. Compare that to our current educational track which has most people achieving their doctorate in their 30s. Performance for the average autodidact would likely be much worse.