פרשת ואתחנן- The Torah Split

The Rambam uses this Passuk for the following lesson:

The Torah says:

רַ֡ק הִשָּׁ֣מֶר לְךָ֩ וּשְׁמֹ֨ר נַפְשְׁךָ֜ מְאֹ֗ד פֶּן־תִּשְׁכַּ֨ח אֶת־הַדְּבָרִ֜ים אֲשֶׁר־רָא֣וּ עֵינֶ֗יךָ וּפֶן־יָס֙וּרוּ֙ מִלְּבָ֣בְךָ֔ כֹּ֖ל יְמֵ֣י חַיֶּ֑יךָ וְהוֹדַעְתָּ֥ם לְבָנֶ֖יךָ וְלִבְנֵ֥י בָנֶֽיךָ׃

But take utmost care and watch yourselves scrupulously, so that you do not forget the things that you saw with your own eyes and so that they do not fade from your mind as long as you live. And make them known to your children and to your children’s children:

עַד אֵימָתַי חַיָּב לִלְמֹד תּוֹרָה עַד יוֹם מוֹתוֹ שֶׁנֶּאֱמַר (דברים ד ט) "וּפֶן יָסוּרוּ מִלְּבָבְךָ כּל יְמֵי חַיֶּיךָ". וְכָל זְמַן שֶׁלֹּא יַעֲסֹק בְּלִמּוּד הוּא שׁוֹכֵחַ:

Until what age in life is one obliged to study the Torah? Even until the day of one's demise; for it is said: "And lest they depart from thy heart all the days of thy life" (Deut. 4.9.) Forsooth, as long as one will not occupy himself with study he forgets what he did study.

Rambam learns one has a chiyuv to learn till the day he dies.

שאל בן דמה בן אחותו של ר' ישמעאל את ר' ישמעאל כגון אני שלמדתי כל התורה כולה מהו ללמוד חכמת יונית קרא עליו המקרא הזה לא ימוש ספר התורה הזה מפיך והגית בו יומם ולילה צא ובדוק שעה שאינה לא מן היום ולא מן הלילה ולמוד בה חכמת יונית

Ben Dama, son of Rabbi Yishmael’s sister, asked Rabbi Yishmael: In the case of one such as I, who has learned the entire Torah, what is the halakha with regard to studying Greek wisdom? Rabbi Yishmael recited this verse about him: “This Torah scroll shall not depart from your mouth, and you shall contemplate in it day and night.” Go and search for an hour that is neither part of the day nor part of the night, and learn Greek wisdom in it.

Why does the Gemora not use our passuk to prove one should learn all day?

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There are two diffrent chiyuvim on learning torah!