Once he chose to disobey ... his disposition against God was confirmed. Before he had the sinful nature he had the propensity to sin (which he eventually did) ... and during that time he was certainly human. I.e., Adam didnt have a sin nature initially, he was fully human, then he disobeyed and had the sin nature ... and was fully human.
I guess what I am saying is the possession of the sin nature is not germane to what it means to be human.
But it clearly states that all “men” since Adam had Adam’s sinful nature. When I say “fully man” it includes Adam’s sinful nature. Keep in mind that Jesus had to be fully man or we would, in our human nature, would have an excuse that Jesus really wasn’t a sinful man like we are and didn’t have the same temptations. I believe he had to have had the same nature but resisted them.