Is סוד ה' ליראיו a reason to believe that Chazal didn't err in science?
IMHO the answer is no.
R' Feldman writes
As the Leshem cited above says, if even regarding matters which are not related to halacha, the Sages say, sod Hashem liyerav, “G-d reveals the secrets of nature to those who fear him,” then certainly there must have been siyata dishmaya (Divine assistance) and even ruach hakodesh (a Divine spirit) assisting the Sages in their redaction of the Oral Law. It is therefore inconceivable, to these opinions, that G-d would have permitted falsities to have been transmitted as Torah She-be-al-peh and not have revealed His secrets to those who fear Him.
In other words we have to believe that Chazal got their science from Torah because how could Hashem have hidden these things from them. Hashem would not have allowed them to transmit falsities as Torah.
However this just moves the question to the next stage. No one denies that the remedies found in Chazal don't work today. R' Feldman himself offers the following answer The most widespread explanation offered for this is nishtanu hatevaim, “nature has changed” - cures that worked in the times of the Talmud are no longer effective.
If we think about it we can ask exactly the same question that he asked above about nishtanu hatevaim. Is it conceivable that Hashem would reveal scientific facts in Torah and then change nature to invalidate that revealed Torah? Moreover, if he did that, would he do so without telling Chazal (after all Chazal in their scientific pronouncements never even hint that things will change)? Would Hashem put Chazal in a position where they wrote things down in the Gemara (e.g. remedies) as part of Torah Shebaal Peh that are no longer true?
The questions that I raise are in IMHO at least as inconceivable as the question R' Feldman asked. In both cases falsities end up in the Torah Shebaal Peh (bottom line, the remedies don't work nowadays). This in essence destroys his whole argument. In other words, just like he can't conceive that Hashem would let them err in science I can't conceive that Hashem would give them scientific facts and then change nature to invalidate those torah revealed scientific facts in any case, and especially without even telling Chazal.
You tell me which is more inconceivable.
1.Chazal could make a mistake in science because they relied on the science of their day
2.Hashem would reveal scientific facts (e.g. remedies) as part of Torah Shebaal Peh M'Sinai, and then change nature so that these Torah scientific facts are no longer true, all without any hint that they will change.
I for one believe that 2 is much more inconceivable.
Bottom line, using his logic, how could it be that hashem didn't reveal the secret that nature was going to change? How could he have allowed Chazal to put remedies and other scientific facts in the gemara that were going to change?