It's purely the fault of those who expected such nonsense. Now, Swallolo-chan is coming after your ass...
The worst thing is that Arakami is best equipped among all Admirals to face huge numbers due to his ability to attack high numbers of people at the same time with his roots. If it was any other Admiral, this would turn into a 1vs1 very fast.
Realistically, GB shouldnt get past these troops but the plot treats him well, everything goes his way.
That means Oda might off-panel all these troops that stand in his way.
I am sure he wont show off Aramaki's best vs those who are a side battle for him.
If Zoro is his actual fight, his best is saved up for that.
This clash might just serve to tire him out a bit to make it fair due to Zoro's condition.