It's not at all clear to me why a T-shaped individual would be better than "square" individual of the same depth (ie something with more overall surface area).
I'd say the assumption is that your square shape would sacrifice breadth for depth across their breadth. The assumption is that time is fixed and you can only learn and master a certain amount of material to a certain degree. At some point you're going to sacrifice something for the other because time is fixed. There's probably another element to consider: memory, learning rate, and rate of change of a given industry. If an industry moves fast your dabbling in that field can useless fairly quickly.
The assumption that somebody needs to have sacrificed breadth for depth is only true in the hypothetical case where you need to choose between two applicants with exactly the same amount of experience. In practice, you will encounter candidates who are just better at everything than the other candidate. That is something experience can give you.
