Per the Chazon Ish, the Rambam in the Mishna Torah doesn't necessarily include in his code those things that aren't expressly stated in the Talmud. In other words, the Rambam could hold of a certain vital carveout but not state it because it is not explicitly contained in the Talmud.
But here comes the confusion - the Mishna Torah is a cogent and highly organized one-stop-shopping code of Halacha. It's job thus is very different than that of the Gemarra.
Here is an example of a life and death carveout that the Rambam did not state, but per the Chazon Ish, holds of anyway. One of the three cardinal sins in Judaism is adultery. So how did a married Esther sleep with Achashverosh?
Tosfot on Sanhedrin 74b states:
דקים ליה דמהני טעמא דקרקע עולם לענין דלא מיחייב' למסור עצמה משום עריות
The Talmud holds that Esther was like the natural ground/passive in the act and thus did not have to give herself over to death ...
The Rambam does not codify this carve out when he discusses the big three mizvot. The Chazon Ish here is quoted as saying (brought in the back of the new edition of the Rav Chayim on Rambam):
"There is no proof that the Rambam did not hold of Tosfot's carveout - because [the Rambam does not cite things that] are not explicit in Talmud."
But this is a life and death carve out and the task of a cogent and highly organized codifier is different than that of the talmud. How are we to tell what the Rambam hold's l'Halacha (and what the halacha is) if there are material gaps in his code?!
And the problem gets worse - the Rambam in his own introduction claims that his code is one-stop-shopping:
עַד שֶׁיִּהְיוּ כָּל הַדִּינִין גְּלוּיִין לַקָּטָן וְלַגָּדוֹל בְּדִין כָּל מִצְוָה וּמִצְוָה, וּבְדִין כָּל הַדְּבָרִים שֶׁתִּקְּנוּ חֲכָמִים וּנְבִיאִים: כְּלָלוֹ שֶׁלַּדָּבָר, כְּדֵי שֶׁלֹּא יְהֶא אָדָם צָרִיךְ לְחִבּוּר אַחֵר בָּעוֹלָם בְּדִין מִדִּינֵי יִשְׂרָאֵל; אֵלָא יִהְיֶה חִבּוּר זֶה מְקַבֵּץ לְתוֹרָה שֶׁבְּעַל פֶּה כֻּלָּהּ, עִם הַתַּקָּנוֹת וְהַמִּנְהָגוֹת וְהַגְּזֵרוֹת שֶׁנַּעֲשׂוּ מִיְּמוֹת מֹשֶׁה רַבֵּנוּ וְעַד חִבּוּר הַתַּלְמוּד, וּכְמוֹ שֶׁפֵּרְשׁוּ לָנוּ הַגְּאוֹנִים בְּכָל חִבּוּרֵיהֶן, שֶׁחִבְּרוּ אַחַר הַתַּלְמוּד. לְפִיכָּךְ קָרָאתִי שֵׁם חִבּוּר זֶה מִשְׁנֵה תּוֹרָה – לְפִי שֶׁאָדָם קוֹרֶא תּוֹרָה שֶׁבִּכְתָב תְּחִלָּה, וְאַחַר כָּךְ קוֹרֶא בְּזֶה, וְיוֹדֵעַ מִמֶּנּוּ תּוֹרָה שֶׁבְּעַל פֶּה כֻּלָּהּ, וְאֵינוּ צָרִיךְ לִקְרוֹת סֵפֶר אַחֵר בֵּינֵיהֶם. [This will make it possible] for all the laws to be revealed to both those of lesser stature and those of greater stature, regarding every single mitzvah, and also all the practices that were ordained by the Sages and the Prophets. To summarize: [The intent of this text is] that a person will not need another text at all with regard to any Jewish law. Rather, this text will be a compilation of the entire Oral Law, including also the ordinances, customs, and decrees that were enacted from the time of Moses, our teacher, until the completion of the Talmud, as were explained by the Geonim in the texts they composed after the Talmud. Therefore, I have called this text, Mishneh Torah ["the second to the Torah," with the intent that] a person should first study the Written Law, and then study this text and comprehend the entire Oral Law from it, without having to study any other text between the two.
