A quick demonstration I sometimes use to tease out this distinction: “imagine a six-sided die. Are you imagining it? Good. How many dots are on the side closest to you?”
If the phenomenal experience was the same and the difference is in the description, everyone should still be able to give a number as their answer, even if they have to use different processes to derive the number from their different descriptions. But aphantasiacs will visibly struggle to answer this question, or give a non-number non-answer like “no side is closest to me” or “depends on which way the die is facing”.
This test implies that phantasiacs immediately lock down the full visual fidelity (the orientation of the die), before you specified it. Why would they? I find myself capable of imagining the die in superposition, same as I don't imagine any particular color it has or any surface it may rest on.
If the phenomenal experience was the same and the difference is in the description, everyone should still be able to give a number as their answer, even if they have to use different processes to derive the number from their different descriptions. But aphantasiacs will visibly struggle to answer this question, or give a non-number non-answer like “no side is closest to me” or “depends on which way the die is facing”.