Is there psak in hashkafa?
The Rambam writes in 3 places in the Perush Hamishnayos (Sanhedrin 10,3, Sotah 3,3, Shavuos 4,1) and in the Sefer Hamitzvos lo taaseh 133 that there is no psak on hashkafa which is not relevant to halacha. The last point is a crucial one, namely the Rambam is talking where there is no practical difference in halacha. Here is a quote from one of the Rambam's (Sotah 3,3 the language is almost the same in all of them)
וכבר אמרתי פעמים רבות כשיש מח' בין החכמים בסברת אמונה אין תכליתו מעשה מן המעשים שאין אומרים שם הלכה כפלוני
As I already wrote many times when there is a dispute between the chachamim on a matter of faith that has no relevance to action we don''t say the hakacha is like ploni
Let us now analyze the Slifkin affair in light of the Rambam.
The creation story/age of the world has absolutely no relevance to halacha, there is no difference in halacha whether we say that it was 6 days of 24 hours, 6 long days, or 6 time periods. The dispute is purely in terms of ideas (hashkafa) and therefore as the Rambam says there is no psak on such a dispute. And therefore R' Slifkin as long as he finds some source (which he has, R' Dessler) is fine.
Whether Chazal made mistakes in science would also seem to not affect halacha at all. As R' Dessler and others said, the halacha doesn't change even if the reason given is wrong in science, we assume that the halacha Chazal had a kabbala on and they made a mistake in the reason. Therefore, again the principle of the Rambam should apply, since there is no practical relevance to halacha there is no psak on such a case. Therefore, we cannot say that there is psak against the Rambam as this is not something that we pasken on.
One note, according to this Rambam there could be a psak regarding the ikkarei haemuna as that is relevant to halacha, who is a kofer. However, in any case, this is not relevant to the Slifkin affair because neither the order of the creation story/age of the world nor whether chazal made mistakes in science is one of the ikarrei haemuna and therefore they are not relevant to halacha.