Apparently, there was more than one different type of power in effect around Legion. One pushed while the other destroyed. The black tentacles can overcome one, but they can't overcome both of them at the same time.
This is the difference between Legion and the black ball. The black ball of flesh has the upper hand in the number of Supreme Powers at its disposal. It has tens of thousands, while they only have five.
It can use anyone Supreme Power needs for whichever difficulty it encounters, and it can use all of them at once. But that's only an advantage in quantity. It still can't overcome the advantage of the quality of power that Legion currently possesses.
Legion can use their Supreme powers together and as one. They are not like the scattered soldiers of the Tyrant amalgam. They are a giant created by the perfect fusion of many soldiers. They have one Origin and can control their power perfectly.
