Brisket and other cuts high in connective tissue (shank, chuck, pork shoulder/butt, chicken dark meat, etc) cook differently than "lean" meats that don't contain connective tissue.
If no connective tissue, then your statement above is correct; "moist" meat is largely based on water content of the meat, and if you keep cooking, it just loses moisture, and gets dry and overcooked. You will never be able to do a 6hr cook on a lean cut like a filet or a pork tenderloin, those are zero connective tissue and the cook is all about managing water content. Cook out the water and they are dry as fuck. Internal temp is of utmost importance here.
Connective tissue breaks down into gelatin after some time and temperature. What is happening in a brisket is first you cook off the water (hence the stall), but then you keep cooking to break down the connective tissue into gelatin. A "moist" piece of brisket is largely gelatin. If you undercook a brisket, you have cooked off all the water, but you have not converted connective tissue to gelatin, resulting in a totally dry piece of meat that is also chewy as fuck. As you continue to cook, the chewiness decreases because the chewy connective tissue is converting into gelatin, which is also increasing the "moist" feeling of the meat.
For low and slow, internal temp is completely irrelevant, as different animals and cuts have differing amounts of connective tissue. Hence the probe test. It's done when it's done.