Prime Hizuren is real just not as strong as people think. He's basically old Hizuren with more stamina.
We've already seen most of Itachi's abilities, so it would be easier to imagine how he would be if he was healthy since he was brought back as an Edo, and saw him fight Nagato and Kabuto. So that gave us a borderline.
For Hiruzen we only saw him use some of his ability when he was old not young, it would be difficult to imagine how many times powerful Prime Hiruzen would since we haven't seen him in combat at young.
Prime Hizuren is real just not as strong as people think. He's basically old Hizuren with more stamina.
both is fanfic....so hard to understand?