That's pretty simplistic to say that's parking the bus. So how is Arsenal getting clean sheets against the non big teams?
No one is saying Holding is world class or anything like that. He's a solid defender that is better than average.
You clearly mentionned against the big teams and I explained why tactically.
You should take a real look at his personnal results also :
Holding has played 17 EPL games this year. His record is 7 wins, 4 draws, 6 losses. He has 8 clean sheets out of these 17 games.
Against the traditionnal top 6, he has 2 wins, 1 draw, 2 losses (2 clean sheets).
His record against non top 6 teams this year is 5 wins, 3 draws and 4 losses. He has 6 clean sheets ouf of those games.
Arsenal's record without Holding is 2 wins and 2 losses.
Sorry, but Holding simply has zero impact on Arsenal's record. He's average.
EDIT : I should add that Arsenal parking the bus is why they have the second best defense in the league.