Don’t see a word there, or anywhere else of which I’m aware, that, “Only descendants of Adam can be saved.”
It’s implied, but I don’t think it is ever openly stated. The implication comes from the fact that Christ’s grace is described as repurchasing us from bondage, and that bondage arose through Adam. It was only the responsibility of your closest relatives or descendants to repurchase you, your family, or your property from bondage.
There are examples of this practice in the Bible, such as when the prophet Jeremiah had the responsibility to repurchase lands from the tribe of Dan, because he was the appointed redeemer for his tribe. He would not have been repurchasing the sold lands for other tribes, because he did not have the authority to act as a redeemer for them.
This is another reason why it was important to see Jesus’ lineage detailed in the Gospels. Not only did his descent need to be without blemish in order for Jesus to be the “spotless lamb” to fulfill the passover offering, once and for all, but He also needed to be of a lineage that was qualified to repurchase Adam’s original debt to God.
To your mind, what was it about Christ's human ancestry that qualified him in this way?