Didn't they all state that Hiruzen was the strongest Shinobi in the Leaf at his time and that he was the strongest in his era? Hiruzen's era would simply mean the Era in between Hashirama Senju and Minato Namikaze, the 1st and 4th Hokage. That means out of Hiruzen and Tobirama, Hiruzen was the stronger one. They also stated that Hiruzen got the nickname God of Shinobi because of his strength, but that was years after Hashirama's death and its also stated that most believed that the stories about Hashirama's strength was mere legends and stories people had made up. If they all though that the stories about how strong Hashirama was when its only natural that they would say that Hiruzen was the strongest Hokage ever since they only knew about the power of 3 Hokgae's.
Minato, Hiruzen and Tobirama. They might have only stated that Hirzuen was the strongest of the 3 in his prime, which makes sense since the apprentice will always surpass the master and Minato died before he could reach his prime. I highly doubt Kishi though about it before he made it, but it can be turned around to make a decent amount of sense if you think about it.
I also saw the statement that Minato during the attack on the Nine-Tails was barely able to transport a Bijuu Bomb from Kurmama while in the war he was able to transport the gigantic one from the Juubi without trouble, the reason for this is because he is a Edo. Since he is a Edo Tensei, Minato doesn't suffer from losing chakra like he did under the attack on the Nine Tails.
