He was using the same type of logic we use in a Kal Vachomer. We don’t know the reason the Mitzva was given but we observe the given parameters and use them as laws. We don’t understand ‘why’ but we understand ‘that’. The Gemara uses this type of ‘Svara’.
Here too, we observe how there is a Chiyuv on a four-cornered garment which Techeles resolves. So we know that for whatever reason, or even for no reason, Techeles ‘fixes’ this garment. Therefore, he asked why would Techeles itself have to be ‘fixed’.