Kakashi is not a ninja to make assumptions .. He assumed he could over power itachi's sharingan in part one U_U .. And how was Sakura more skilled then Sasori in human anatomy/biology, Sasori was the sole person able to take a human body and transform it into a deadly puppet .. that alone is a great anatomy accomplishment
He had lack of intel at that time.
MS techs were supposed to be very rare techniques at that time. Even jiraiya/chiyo didn't knew about them despite of their years of experience.
If you look at his words, you can see he is quite certain of what he speaks.
Making of human puppet is correct. But the thing which make sasori stand out of from the rest is his deadly poison which even lady chiyo could not find cure of, because she wasn't better than sasori at this stuff. Sakura is better than him, which is why she found the cure.
anyway we are going off topic i suppose.