The alternative reasoning proceeds as follows: if something were genuinely comparable to a subject, a reciprocal relation of likeness would arise between the subject and its supposed counterpart. That relation could be examined under the phrase “the like of a like.”
Denying this necessary relation removes the supposition of an original counterpart as well. The intended assertion is that nothing is comparable to God; “a likeness of a likeness” is not being introduced as an existing external being.
The rhetorical structure is negation of an underlying supposition by negating something that would necessarily follow from it.