The text uses garments to show that resemblance may concern something more essential than colour or softness. A shirt and another garment can be considered under the shared idea of clothing or material.
Qamīṣ: A shirt considered as a garment.
Thawb: Cloth or an article of dress.
If the comparison concerns being clothing, it may involve a kind or a general essential constituent. If it concerns only whiteness, whiteness is an attribute distinct from what makes either item the kind of garment it is.
Consequently, the same pair can admit different classifications depending on the intended shared respect, not merely the nouns used.