The book of Deuteronomy is not a second telling of the Sinai Covenant, but a different covenant altogether.

Deut 29:1 reads, “These are the words of the covnenant which the LORD commanded Moses to make with the sons of ISrael in the land of Moab, besides the covenant which He had made with them at Horeb (i.e. Sinai).”

Also, it seems that this Moab Covenant is truly “Mosaic” since it is made by Moses. Here in Deuteronomy at Moab, a divine theophany like the one that occurred at Mount Horeb/Sinai is lacking.

This would explain why Christ said, “Moses allowed you to divorce.”