Korach

16,1 Rashi- Korach and his followers came to Moshe with four cornered garments made out of Techeiles and they asked Moshe is this garment chayav in Tzitzis or patur? Moshe responded that its still chayav in Tzitzis. Immediately they began laughing at him and said if a regular beged has one string of techeilis that paturs the entire beged, so this garment which is completely techeiles should for sure be patur. Why doesn't Korach agree that its chayav in Tzitzis? All it should be patur on based on Korach's Kal Vechomer is from techeiles! So why did he ask whether its chayav in Tzitzis, not whether its chayav in Techeiles?