Charles Ellicott Commentary Deuteronomy 1:18

"And I commanded you at that time all the things which ye should do." — Deuteronomy 1:18 (ASV)

And I commanded you at that time all the things which ye should do. — “At that time,” i.e., after your departure from Horeb. This is to say that the exhortations given in Deuteronomy had already been given on the way from Sinai to Kadesh-barnea. (Compare what has been said above on the first two verses of this chapter.) This verse goes far to justify the view taken there.