It's not a myth. Every cell of an organism has a certain degree of 'intelligence'. Roots respond to how much space they have available, and grow accordingly. What many people consider root bound is not anywhere near actually being root bound. The plant will 'show you' when it's root bound. Just because roots are coming out of holes in the container is not a reliable indicator. There is a direct relationship between root mass and the amount of (above ground) plant mass the root system is capable of supporting(and the shape of the roots influences the shape on the plant). The plant will stop growing(or be greatly stunted), basically, when that point is reached. This is true root bound.
In nature adaptation is a necessity. I've seen fields of wild hemp, literally with nearly no gaps between stalks(probably ten's of thousands of plants, growing a ways up from a river bank), and they get very tall(up to 16 feet in the mid-west, which is pretty impressive), with very deep roots, but they're not allowed to spread much due to competition(under and above ground). So the plant responds accordingly and focuses on vertical expansion.
Then you have plants like fdd's, which are fuller than some Christmas trees. They have all the room they could ever desire to vertically and horizontally expand(above and below ground). This is the best way, if you have the space available.
