This interestingly phrased question of Korach appears in two other sources.
Badmibar Rabbah 18:3
קָפַץ קֹרַח וְאָמַר לְמשֶׁה טַלִּית שֶׁכֻּלָּהּ תְּכֵלֶת מַהוּ שֶׁתְּהֵא פְּטוּרָה מִן הַצִּיצִית, אָמַר לוֹ חַיֶּיבֶת בְּצִיצִית. אָמַר לוֹ קֹרַח טַלִית שֶׁכֻּלָּהּ תְּכֵלֶת אֵין פּוֹטֶרֶת עַצְמָהּ, אַרְבָּעָה חוּטִין פּוֹטְרוֹת אוֹתָהּ
As well as Yerushalmi Sanhedrin 10:1
עמד ועשה טלית שכולן תכלת אתא גבי משה א"ל משה רבינו טלית שכלה תכלת מה שתהא חייבת בציצית אמר לו חייבת דכתיב גדילים תעשה לך וגו'
In both places as well it says Korach asked if this garment is obligated in tzitzis, where seemingly it being made of techeiles shouldn't make a difference to if it needs tzitzis or not. Your question is actually something that many commentaries were bothered by (Mizrachi to Numbers 16:1; Eitz Yosef (quoting Zayis Ra’anan), Maharzu, and Chiddushei HaRashash to Bamidbar Rabbah; Eitz Yosef (quoting Zayis Ra’anan) and Biur HaAmarim to Midrash Tanchuma Korach § 3; and it seems the Korbon HaEidah to Yerushalmi Sanhedrin).
Therefore, they explain that Korach wanted to know if he needed to dye the tzitzis in his garment techeiles or is the garment being that color sufficient. Moshe responded that he still needed to make some of the strings techeiles. This explanation fits better with Korach’s subsequent question, which is similar in nature: Does a house full of scrolls need a scroll on the doorpost, i.e. a mezuzah. What they have in common is I have X, why would I need another X. According to the plain meaning of the words, the two questions aren't as similar.
However, the Kesav Sofer ad. loc. understands the Midrash according to its literal wording. He seems to interpret it like @mevaqesh suggested, that Korach felt that a garment full of techeiles accomplished the purpose of the mitzvah of tzitzis, so there was no point in adding tzitzis to it.
Now, this sounds like Korach is doresh ta'amah dekra, that he applies the reason behind the mitzvah to its laws. There are late achronim who explicitly suggest that this was Korach's opinion. It's actually a dispute if we are doresh ta'amah dekra, and some of these achronim suggest that this was the dispute between Korach and Moshe, where the latter held we aren't doresh ta'amah dekra. These achronim are, inter alia, Avnei Eliyahu parshas Chukas s.v. ובזה י"ל [#1], Harei BaShamayim ad. loc. s.v. במדרש מה ראה [#2], Chavatzeles HaSharon parshas Ki Seitzei s.v. וסמך נחמד (although he provides an alternative explanation), and Chakel Yitzchak parshas Korach s.v. והנה לר' שמעון. If you look at these sources inside, you'll see some go with the literal understanding of the Midrash, and some go with the explanation of the above commentaries. Either way, it seems Korach was doresh ta'amah dekrah.