I would say that Jesus being the cornerstone kinda says “he” is the foundation for “the assembly of the saved ones”.
Without the divine sacrifice of the Son of God, who is the Messiah of Israel, there is no salvation ... and then, as a result ... no assembly of the saved ones.
The “rock” is that Jesus “is the Messiah” ... otherwise no one is saved.
Ephesians is very clear. Perhaps you will argue that Peter was not an apostle or the Gentiles are not included in the church that Messiah is building.