As the JWs believe Jesus was born of the virgin Mary by the Holy Spirit, they maintain that He was born without Adamic sin. Romans 3:23 is speaking of all humans who have sinned after the nature of Adam (5:12-20). Jesus was born sinless - perfect.
Now, the JWs limit this to 'just' being a perfect human. They deny that Jesus was God incarnate. They try to liken the perfect man, Jesus, to the perfect man Adam before he sinned. This is to bolster their doctrine that Jesus was the exact equivalent of Adam, no more and no less, in order to 'buy back' what Adam lost (not heaven, but living forever on a paradise Earth.)
This is important to bear in mind, because the problem with JWs saying Jesus did not fall under Romans 3:23 is that if He was only equivalent to Adam, Jesus' death could only have counter-balanced the sin of one man. No mere human, not even a perfect one, could become "the Lamb of God that takes away the sin of the world" (John 1:29). To do that, He would have to have the magnitude of God! That is why Romans 5 points out the differences between Adam and Jesus; "But the gift is not like the trespass... Again, the gift of God is not like the result of the one man's sin..." (verses 15-20)
The JWs theology ends up admitting that Jesus couldn't die to allow anybody into heaven! They have to resort to suggesting that those with the heavenly calling have to "give up" a supposed "right" to everlasting right on a paradise Earth in exchange for their ticket to heaven. This is because they maintain that Jesus only died in order to restore what Adam lost for humanity. All of this boils back to their warped view that a created angel (Michael) was changed into a perfect human baby in Mary's womb and was only human, losing that humanity after dying, and now being a spirit creation. They actually have Jehovah creating an angel, then creating a human baby, then creating a spirit creature. They try to keep continuity between these three creations, but biblically, this is heretical.