The qualification “the concept itself” matters because multiplicity may fail for a reason external to the meaning, even when that meaning would not itself prohibit multiple instances.
Observing just one familiar sun in the sky at some historical moment does not prove that the general concept of a sun rules out other such celestial bodies.
By contrast, the identity of one determinate individual excludes a distinct person being numerically that very individual. Thus restricted external existence is not the same as intrinsic particularity.