In the musaf of Rosh Hashana, in the Zichronot section, we say
וְגַם אֶת נֹֽחַ בְּאַהֲבָה זָכַֽרְתָּ. וַתִּפְקְדֵֽהוּ בִּדְבַר יְשׁוּעָה וְרַחֲמִים בַּהֲבִיאֲךָ אֶת מֵי הַמַּבּוּל לְשַׁחֵת כָּל בָּשָׂר מִפְּנֵי רֹֽעַ מַעַלְלֵיהֶם. עַל כֵּן זִכְרוֹנוֹ בָּא לְפָנֶֽיךָ יְהֹוָה אֱלֹהֵֽינוּ לְהַרְבּוֹת זַרְעוֹ כְּעַפְרוֹת תֵּבֵל. וְצֶאֱצָאָיו כְּחוֹל הַיָּם. כַּכָּתוּב בְּתוֹרָתֶֽךָ וַיִּזְכֹּר אֱלֹהִים אֶת נֹחַ וְאֵת כָּל הַחַיָּה וְאֶת כָּל הַבְּהֵמָה אֲשֶׁר אִתּוֹ בַּתֵּבָה וַיַּעֲבֵר אֱלֹהִים רֽוּחַ עַל הָאָרֶץ וַיָּשֹֽׁכּוּ הַמָּֽיִם.
And No’ach too, You remembered with love, and [therefore] decreed for him a promise of deliverance and compassion, when You brought the flood-waters to destroy all flesh because of the wickedness of their deeds. Therefore, his remembrance came before You, Adonoy, our God, to multiply his seed like the dust of the earth, and his descendants as the sand of the sea; as it is written in Your Torah; “And God remembered No’ach and all the beasts and all the cattle that were with him in the Ark, and God caused a wind to pass over the earth, and the waters were calmed.”
But in the text, I can't find that Hashem made that promise to Noach. The pasuk which follows (and which appears to be cited to support the claim about multiplying seed etc) just says that Noach and everything/one in the Ark was remembered but nothing about dust of the earth or sand. The Ibn Ezra (and many others) on Bereishit 6:18 says that the brit between Hashem and Noach was simply that Hashem would save Noach's life. The promises about becoming as multiplied as dirt and sand appear later and were made to Avraham (22:17).
So what exactly is this reference all about?