The word "word" is a metonym for the promise that God used the words to make. Alternate translation: "But we should not think that God has failed to keep his promises" or "We should know that God has kept his promises" (See [[rc://en/ta/man/jit/figs-metonymy]])
God did not make his promises to all the physical descendants of Israel (or Jacob), but to his spiritual descendants, that is, those who trust in Jesus.