I respectfully disagree with this statement, IMO healthy roots are unrestricted roots.
But, there's more than one way to skin a cat.
The best way to learn is through trial and error, and to take people's opinions(including mine) with a grain of salt.
I start in two-gallon pots and add more amendments to the bottom third of the soil. So that for the first couple of weeks the seedlings are growing in less nutrient-rich soil, which is best for new seedlings. But, as the seedlings grow and their nutrient needs increase the plant's roots will reach down to the bottom where the soil contains the higher levels of nutrients that plants require at that stage of growth.
Good job taking a grow to harvest, and good luck in the future.