and in perek 8,
and on 17:19, Ibn Ezra writes:
מאד מאד -
פעמים שאין לרוב ממנו.
וכן אחר שאמר: ויכסו כל ההרים הגבוהים.
למה אמרו ויכסו ההרים?
וכן פי' כל הר גבוה, כסה במים וחמש עשרה אמה כסו ההרים הנזכרים, ואנחנו נאמין בדברי אלהינו ונעזוב הבלי בני האדם, שיאמרו על הר אחד גבוה שהוא במלכות יון:
That is, they say something about a certain high mountain in the kingdom of Greece. Now, if they mean Olympus, the highest mountain in Greece, it is not higher than Ararat. But maybe they meant a mountain in some other area in the greater kingdom of Greece. And indeed there are higher mountains than Ararat in the world, and in Europe. Ararat is 3896 m, while Elbrus is 5642 m.
Apparently, the claim is that it would not have covered the higher mountain, for some reason. Presumably because the water only covered 15 cubits above Ararat, and this mountain is higher than that. Or that the revealing the tops of the mountains was after the ark rested on Ararat.
Perhaps we can take Ibn Ezra's statement that "we believe the words of our God and discard the vanities of those who speak of a high mountain in the kingdom of Greece" as rejecting the facts of the situation -- that there is no such higher mountain. (If so, we know that indeed there are higher mountains.)
Or/and probably he is claiming that since the verse states all the mountains, it would include such a mountain there, somehow. Though he does not explain precisely how this works out with the pesukim.
Rabbi Shmuel Motot says that one can explain it as the waters being much higher, initially. Then, they reduced and the teva rested on Ararat. And then when it says the tops of the mountains were seen, after that, it refers to those other mountains which were smaller than Ararat. And Mechokekei Yehuda says similarly.
And see Shadal, who writes:
אז חדל הגשם, ועם כל זה נשארו המים גבוהים ומכסים ההרים היותר גבוהים עד י"ז בתשרי (למטה ח' ד' ), אז נשארו ראשי קצת ההרים מגולים ותנח התבה על הרי אררט שאינם היותר גבוהים, ובאחד בטבת (שם ה' ) ניגלו ראשי הרי אררט ושאר הרים הבלתי גבוהים כל כך
I was tempted to say that in
|כ חֲמֵשׁ עֶשְׂרֵה אַמָּה מִלְמַעְלָה, גָּבְרוּ הַמָּיִם; וַיְכֻסּוּ, הֶהָרִים.||20 Fifteen cubits upward did the waters prevail; and the mountains were covered.|
15 cubits was the entire height of the water, such that it reached and covered part of all the mountains. But this conflicts with the previous pasuk somewhat, and greatly with the next perek which has the mountain tops revealed.