Do you believe for a second that God did not consider the covenant child of Abraham, i.e. Ishmael in any sort of fashion? Obviously, from scripture, we can see that God respected His command of covenant even when it came to those reprobate within the external portion of said covenant.
The way the apostle is using the term is in respect to how the covenant keeping parents are obedient in pacing that sign upon their children; much akin to the holy goblets used in the temple, separated unto the Lord and then later ransacked by blasphemers who later used it to get drunk with or melted it down to make idols. It is God who sets things apart for His purpose.
Being in covenant by sign says much; in the compound sense this reprobate child is at enmity; in the divided, he is holy unto the purposes of God.
All covenant children are commanded to be holy; to pray, repent, receive, accept. They are reminded often to ‘perfect their baptism’. Reprobation ultimately rears it’s ugly head. Before Judas fell, do you believe the apostles or Judas himself believed he was reprobate?
Gen 17:19 Then God said: “No, Sarah your wife shall bear you a son, and you shall call his name Isaac; I will establish My covenant with him for an everlasting covenant, and with his descendants after him. 20 And as for Ishmael, I have heard you. Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly. He shall beget twelve princes, and I will make him a great nation. 21 But My covenant I will establish with Isaac, whom Sarah shall bear to you at this set time next year.” 22 Then He finished talking with him, and God went up from Abraham.
You will notice that ishmael was only 13 yrs old when God made this promise-to a reprobate!
23 So Abraham took Ishmael his son, all who were born in his house and all who were bought with his money, every male among the men of Abraham’s house, and circumcised the flesh of their foreskins that very same day, as God had said to him. 24 Abraham was ninety-nine years old when he was circumcised in the flesh of his foreskin. 25 And Ishmael his son was thirteen years old when he was circumcised in the flesh of his foreskin. 26 That very same day Abraham was circumcised, and his son Ishmael; 27 and all the men of his house, born in the house or bought with money from a foreigner, were circumcised with him.
God is a God of covenant. Granted, Ishmael’s blessings were Earthly; none the less, they were ‘blessings’ of God unto the reprobate, and that secondary to covenant promises.