What you are bringing from the Netziv is not really relevant. It can be both true that the Rambam was influenced by Greco-Islamic philosophy and also true that once he writes something it becomes Torah.
Again, you said that the Gra can say the Rambam was influenced by philosophy but we can’t. What is the source for that?
Ok, so I went looking for Rabbi Soloveitchik’s writings on this topic and I found a transcript of a speech he gave in 1975 on jewishlinknews about it which is what I think you are referring to. I didn’t have a chance to read the whole thing, however from just skimming it I see that it is talking about histocrizing halacha but its primary focus is on chazal, not rishonim or achronim. Here’s a key passage:
“The Rambam in perek gimel of Hilchos Teshuva, halacha cheis, «וכן הכופר בפרושה והוא תורה שבעל פה והמכחיש מגידיה כגון צדוק ובייתוס»[8]. It is very strange; I wanted to discuss it with my father zichrono leveracha. If he says that whoever denies the truthfulness or the authenticity of Torah SheBe’al Peh is a Tzeduki, why did he add והמכחיש מגידיה – «Whoever denies the authority of the scholars of the masora»? So apparently, the Rambam says that under the category of kofrim baTorah are classified not only those who deny, that for instance, nisuch hamayim is required or arava she’ba’Mikdash is required, or they deny the Torah SheBe’al Peh; there is no doubt about it, but moreover, even those who admit the truthfulness of the Torah SheBe’al Peh but they are critical of chachmei Chazal as personalities, who find fault in chachmei Chazal, fault in their character Rachmana litzlan, or in their behavior, in their conduct, say that chochmei Chazal were prejudiced, which actually has no impact upon the halacha – nevertheless, he is to be considered as a kofer. «וכן הכופר בפרושה והוא תורה שבעל פה והמכחיש מגידיה» – what does it mean «והמכחיש מגידיה»? He denies the perfection and the truthfulness of chachmei Chazal – not of the Torah, again, but of the chachmei Chazal as personalities, as real personae, as far as their character is concerned, their philosophy is concerned, their outlook on the world is concerned.”