Drill Run is actually more common than Knock Off but the point is that they can safely switch into the most spammable attacks of Mega Beedrill.
Similar to Landorus: Celebi loses to Sludge Wave which has over 50% usage, Gengar doesn't switch into Psychic and Knock Off and Rotom-W also can't switch into a Focus Blast or Psychic, yet they are all considered safe switch ins. Otherwise there would only be 2 or 3 pokemon in the list of switch ins because all of those SI (except maybe 2 or 3 pokemon) lose to the right coverage move.
Similar to Landorus: Celebi loses to Sludge Wave which has over 50% usage, Gengar doesn't switch into Psychic and Knock Off and Rotom-W also can't switch into a Focus Blast or Psychic, yet they are all considered safe switch ins. Otherwise there would only be 2 or 3 pokemon in the list of switch ins because all of those SI (except maybe 2 or 3 pokemon) lose to the right coverage move.