Here "I" refers to Moses. The phrase "at that time" refers to when the Israelites were at Horeb, which is the same as Mount Sinai. AT: "Also, when we were at Horeb, I spoke to you" (See: [[rc://en/ta/man/translate/figs-explicit]])
This is an exaggeration to show that God has been faithful to his promise by making it so that there are lots of Israelites. (See: [[rc://en/ta/man/translate/figs-hyperbole]])