I assume you are referring to this statement by PAllen?
"Is determined by" in that statement does not mean "is caused by". It merely refers to the fact that the event horizon is *defined* by which null curves can reach future null infinity, and which can't.
Ok, but PAllen said that the Event Horizon can form before all the matter is within its Schwarzschild Radius, and as I pointed out, an external event could prevent the collapse from completing, so the EH has to "know" about what external events are "going to" happen when it decides whether to come into existence or not!
The future null infinity has not been reached when the EH forms, and in the situation I described it is indeterminate until all the mass is within the SR, so the existence of the EH is similarly indeterminate.