This doesn't work since افترض here is similar to ظن and the sentence is still incomplete, take a look at this:replace it with a مصدر and say أفترض فهمك ما حدث.
This is the first time I hear this. It never occurred to me, but it seems to make some sense as the modern meaning of افترض can work in this way.This doesn't work since افترض here is similar to ظن