My a/c unit is in the attic under a slate roof. The climate here in south Texas is very hot and very humid. I am generating a lot of condensation in that attic despite having a dehumidifier there. The advice which I am getting is to seal the soffets and any other openings in the attic, because the attic is too cool (? ) This seems counter intuitive. How to proceed?

I think you need to ventilate the attic MORE not less.