Exactly
So Mercury does rotate and does have a "day", even if it's 2/3 of a "year" long.
If Mercury WAS tidally locked, the temperature imbalance would be even more extreme between the permanent daytime solar heat and the frigid cold of interstellar space, on the dark side. Something like 1000 K vs 10 K, leading to lithospheric thicknesses of 3:1.
If Mercury was even closer to the Sun, it would be guaranteed to be tide locked and would be in interesting planet indeed. The Solar-facing hemisphere would be molten, while the darkside would have glaciers (or at least ice pockets)!
Fire and Ice!