I agree with that but it doesn't explain how heavenly Jerusalem is the mother of us all. Here is an example of a member of heavenly Jerusalem travailing to birth Christ in a believer. Christ is birthed through preaching the word by a person located on this earth.
Galatians 4:19
My little children, of whom I travail in birth again until Christ be formed in you,
We are born again (birthed) through Christ as we hear the gospel of the Kingdom of God and according to grace through faith we believe. It is in that manner that whosoever has been born again are children of heavenly Jerusalem.
The church of Galatia believed there was still merit before God under the Old Law. They had learned otherwise through preaching by Paul. Those who had been under the Law had been redeemed becoming sons according to adoption through the Spirit of Christ. They had gone from being servants to the Law, and heirs of God through faith in Christ.
Galatians 4:5 (KJV) To redeem them that were under the law, that we might receive the adoption of sons.
Galatians 4:6 (KJV) And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.
Galatians 4:7 (KJV) Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ.
For whatever reason some among them believed after having become heirs of God with Christ they should turn again to the Law that brought them under bondage. After having been born again through Paul's preaching, he thought it necessary for them to once again recall the travail or difficulties that come when faithfully preaching the gospel to them, now it seemed he needed to once again preach the gospel until it became apparent that Christ was formed in them. IOW proclaiming to have been born again and an heir of God with Christ, but turning back to the Law, shows their profession of faith did not outwardly show them as a new creation that is Christ like through His Spirit within them.
Galatians 4:9 (KJV) But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?
Galatians 4:19 (KJV) My little children, of whom I travail in birth again until Christ be formed in you,