Because it's the conclusion that makes the most sense based on what we've seen in the manga.This authors intent still doesn't make sense though. If we go by seeing at what age Hashirama died. We can see he died in his mid 30s. Meaning Tsunade was not that much older at that time yet. How are we supposed to ignore such a loop hole and claim Hashirama compared Sakura to Tsunade in her prime? Seriously use your brain mister!
-Sakura has shown better strength feats than Adult Tsunade with all that experience.
-Subsequently, there's no way that Tsunade as a toddler could have "comparable" strength to her. It makes no sense
It's painfully obvious that Kishi meant to compare Sakura to Tsunade in his prime.