I'm in the under cook camp too. that therm probe was probably in a fat pocket. Injections will not add moisture to any piece of meat as the meat cooks it tightens up pushing out any water based solutions. My questions are did it probe tender in the thickest part of the Flat, if not it wasn't done?! What temp where you cooking at on the Grate? If it was under 300 it aint done!
BBQ RULES FOR SUCCESS
YOU CAN NOT COOK GREAT BBQ ON A CONSISTENT BASIS BY COOKING TO AN INTERNAL TEMPERATURE OR BY TIME ( XXX MIN PER LB) YOU MUST COOK BY FEEL! For Brisket it must pass the poke test(probe like soft butter in the thickest part of the Flat) Ribs pass the Bend Test, Pork Butts when the bone wiggles loose. These are the only reliable methods to ensure that your cook will be a success. There is one exception to these rules and that is Poultry which must achieve and internal temp of 170 deg in the thickest part of the thigh and 165 in the breast.
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC