Moses said the Sinai covenant was made with everyone he was speaking to in Deuteronomy 5:1-5, but many of them hadn't been born yet when Israel stood at the foot of Mt. Sinai. The covenant is inherited by birth or adoption. All those who come later and are born into Israel or grafted in by faith are considered to have stood at Sinai with that first generation of former slaves. The collective nation of Israel stood face-to-face with God at Sinai, and so did every individual Israelite who came after them, whether native and grafted in.