Calculating the Hebrew calendar

I saw a rabbi claim that one of the strongest proofs that the words of Chazal are connected to the Holy Spirit and “the words of the living God” is Rabban Gamliel’s calculation of the Hebrew calendar (the holy month).
What scientists only know today (that a month is 28.315 days) the sages already knew in their holy spirit in the past.
What do you think of this proof as proof of the total truth of the words of the sages or something like that?
I ask because I have also seen places where the Sages were completely scientifically wrong, (Heat is a saw in the sky/ A louse is not a cow and a rabbi, etc.) So sometimes the Holy Spirit worked and sometimes not?
What do you think about this matter? thanks.