The key phrase is "it seems as though." It seemed as though God had failed to keep his covenant with Israel when so many Jews rejected Jesus as the Messiah. It seemed as though God abandoned His promises to the Jews as He now is focusing on the Gentiles and the church.
The objectors didn’t realize that God was faithful to every individual Jew who believed in Jesus. His promises are contingent upon an individual's faith in Him. God is faithful. A Jew is not saved because he is a Jew but because of his faith in Christ.
So, God did keep His word. It just seemed as though He failed because of a wrong assumption that God would bring salvation to the entire nation of Israel just because they were Jews by physical descent irregardless of faith in Christ.
God is faithful. He keeps His promise of salvation to every person who believes on the Lord Jesus Christ. We really can have absolute confidence in God and His word.