It seems to imply that a mediator is required for a covenant of two. In the 2015 edition helpful. But God himself made the covenant with Abraham without an angel as a mediator, thus the covenant of promise is shown as more intimate & greater. But there is nothing to suggest a mediator is helpful for a covenant. A mediator implies indirect or a second hand contract.
Now a mediator is helpful if more than one party must reach an agreement. But God, who is one, did not use a mediator when he gave his promise to Abraham
The recepient of the promise in question is clear and understood from verse 16, 18 etc.
ESV
Acts 7:17: “"But as the time of the promise drew near, which God had granted to Abraham, the people increased and multiplied in Egypt”
Romans 4:13: “For the promise to Abraham and his offspring that he
would be heir of the world did not come through the law but through
the righteousness of faith.”
Romans 4:16: “That is why it depends on faith, in order that the
promise may rest on grace and be guaranteed to all his offspring-not
only to the adherent of the law but also to the one who shares the
faith of Abraham, who is the father of us all,”
Galatians 3:16: “Now the promises were made to Abraham and to his
offspring. It does not say, "And to offsprings," referring to many,
but referring to one, "And to your offspring," who is Christ.”
Galatians 3:18: “For if the inheritance comes by the law, it no longer
comes by promise; but God gave it to Abraham by a promise.”
Hebrews 6:13: “For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself,”