She could have been strong back than, after all her power is natural, and considering hashi was also said to have super strength, this concept doesn't seem impossible if you ask me
My assumption.... since Kishi made Minato compare sakura to kushina (which threw me off first) I guess he only sees it fair to let Hashi compare her to tsunade. To further her importance or maybe to give her a stronger identity among the story's female ninjas.
Tsunade became Genin at the age of six.... Plus Mito,Tsunade and now Sakura have the same forehead seal.... May be Hashi felt Sakura's seal power was more than Mito's and Tsunade's(By estimation in Tsunade's case).... =D =D
perhaps tsunade created her super strength tech(the normal oone, not Byakugou and creation rebirth) at a pretty young age.\young enough that Hashi knew it.