What makes sous vide brisket tender?

The low, constant temperature balanced the hydrolysis of collagen into gelatin with keeping the other meat proteins from over coagulating.

Based on a cursory search on the internet, hydrolysis of collagen into gelatin begins at roughly 140°F, although it really gets going at 160°-180°F. Had the cook left the brisket in for longer, it probably would have become more tender, but some hydrolysis likely had taken place. This hydrolysis might have been enhanced by the addition of acid that served as a catalyst to the hydrolysis.

Additionally, the meat was kept at 140°F, i.e. at medium. This means that the cook kept the proteins in the beef from coagulating to the point that they squeezed water out of the meet. Usually, stewed or slow cooked meats must be cooked longer and at higher temperatures (e.g. in a pot roast, a barbecue, or a braise) because not only does the cook need to hydrolyze the collagen to avoid toughness from the collagen, but also needs to hydrolyze additional collagen to make up for the dryness that results from the fact that most stewed and braised meats are, technically, overcooked. The 140°F for four hours manages to hydrolyze some of the collagen, but it was probably enough because the meat was not overcooked. Note, however, that leaving the meat in an immersion cooker for longer will hydrolyze more collagen but will not overcook the meat, meaning that it will be even more tender.

Then, the cook can further prevent toughness by cutting the meat very thin and across the grain. This mean that any meat fibers that are tough won't be very long and can effectively be swallowed whole.