In his long NBA career, spanning from 1991 to 2009, Mutombo won four defensive player of the year awards, made six All-Defensive teams, led the league twice in defensive rebounds (finishing 11th all-time) and recorded more blocked shots than any player in history other than
Hakeem Olajuwon. (Of note, blocks weren't an official stat until the 1973-74 season, shortchanging many greats, including Kareem Abdul-Jabbar).
Clearly, the box score numbers and the award voters in his day pegged him as an all-time great defender.