Both can be correct.
Under Garrett, and it was similar with Parcells, they drafted to positional prototypes in terms of height/weight/speed/scheme fit. This reduced the Cowboys overall draft board to around 120-150 prospects that would be considered to be drafted in any given cycle. For example, big NT prospects likely were not on the draft list under Garrett due to them playing a 4-3 under system that relied more on smaller, quicker tackles than a player who is a two gapper. Kris Richard didn't want cornerbacks who were shorter than 6 feet tall, and had less than 32" arms. This reduced the number of corners that were on their board. With this in mind, it's possible to take the best player available as the draft approach, with the qualifier being the BPA that is actually on the draft board.
McCarthy clearly is being driven less by positional/schematic prototypes, and more by pure talent. As a result, the draft pool becomes larger. BPA is still accurate, but it encompasses a wider array of players.