But they certainly can talk to any future trade partner that they wish.
The EU rules were not “all about trade” even from the beginning. The 1957 Treaty of Rome made it clear that the intent was for a political union, with the mention of the “ever-closer union” and “ensur(ing) the economic and social progress of their countries by common action in eliminating the barriers which divide Europe” in the preamble. Said treaty also established the common agricultural policy and made mention of a “common commercial policy” in the preamble.