With 20.5 sacks, Donald officially surpasses the NFL record for sacks by a DT, which was set at 18 by Keith Millard in 1989.
What about the unofficial record? It;s Joe Klecko with 20.5 in 1981. That’s here: http://theramshuddle.com/topic/nfl-sack-leaders-including-the-hidden-years/
Using the unofficial record, of the 22 players with 18 or more sacks in a season through all NFL history, only 3 are DTs–Donald, Klecko, and Millard.