1) There's a certain aspect of leadership, namely the ability to rapidly make a decision and stick to it, that TJs are better at than other types. So, in that sense, it's probably not a bad thing that there's a plethora of TJs in those rapid-fire decision-making positions. Now, unfortunately (at least on some level) there are a lot of other roles that are often wrapped up into the managerial position, including some HR-type responsibilities, that FJs would probably be better suited to.
2) The problem with these kinds of "studies" is that they're extremely unreliable. Tests are not reliable indicators of type, so whatever data they got on their study group's types may not be highly accurate. That's huge because it could mean that all of their "statistics" on the presence or absence of certain types in managerial positions could be off.