In Hilchos Ishus 1:2, the Rambam writes:
לִקּוּחִין אֵלּוּ מִצְוַת עֲשֵׂה שֶׁל תּוֹרָה הֵם. וּבְאֶחָד מִשְּׁלֹשָׁה דְּבָרִים אֵלּוּ הָאִשָּׁה נִקְנֵית. בְּכֶסֶף. אוֹ בִּשְׁטָר. אוֹ בְּבִיאָה. בְּבִיאָה וּבִשְׁטָר מֵהַתּוֹרָה. וּבְכֶסֶף מִדִּבְרֵי סוֹפְרִים.
And taking a wife as such is a positive commandment of the Torah. And a woman is acquired through three means: money, a contract, or through intercourse. Marriage through intercourse and by contract is from the Torah, and by money is from the words of the Scribes
They explain that the Rambam distinguishes between laws that are clearly written in the Torah (or are derived from a straightforward reading of the Torah) and those laws that are derived from some of the hermeneutic devices applied by the Sages. Halakhot derived in that manner are referred to by the Rambam as divrei soferim even though they have the same level of seriousness and severity as laws that are clearly learned from the Torah.
My question TL;DR:
Why did the the Rambam say shtar is from the Torah if (like kesef) it ALSO requires a limmud?
Namely, if shtar is not explicitly written in the Torah and its acceptability is learnt out via a hekesh ("ויצאה והיתה" Kiddushin 9b ) AND following the opinions that hekesh is considered to be a part of one of the 13 Hermeneutical principles (see "במנין המידות" in this link), why didn't the Rambam also say shtar is "divrei sofrim?"