One might mistakenly argue that a universal is a single concept in the mind, and therefore must itself be particular. But having one understood meaning is not the same as being applicable to only one external individual.
“Human” is one intelligible meaning and yet is truly predicable of both Zayd and ʿAmr. The identifying concept of Zayd, by contrast, does not apply as that same individual identity to ʿAmr.
Thus the unity of a mental concept does not imply particularity. The criterion is the concept's capacity for common predication.