The distance of 20 parsecs refers to the length of the plotted maze through the maelstrom. The kessel run includes the lengthy maze through the maelstrum
Solo took a short cut out through the side of the maze which was more direct and thus cut the distance by about 8 parsecs.
It was a cheat because he used an unregulated path through the maelstrom so dangerous that even smugglers would not try it.
At least that is a reinterpretation of the jargon.
Kinda like the marathoner who wins by taking a car for all but the first and last 1000 meters... of a race.
Horsecrap. The script said “parsecs” so his talking monkey said “parsecs.”
And it said “parsecs” because George Lucas knows diddly squat about astronomy.