Tough and dry is almost 100% guaranteed undercooked, and you may have only missed it by a couple of degrees. There is a pretty small window for a good flat - I'd say no more than 3-4 degrees separates a good flat from a dry tough one to a dry (or maybe moist if it's well marbled) falling apart flat. I don't want mine dry (undercooked) or falling apart (overcooked).....
Temperature probes are just a tool for me that tell me when to START probing with a wooden skewer (I used to use toothpicks or chopsticks from the chinese restaurant, but bought a package of 100 wooden skewers about 5 years ago and still have about 75 of them lol, they are sized between a toothpick and a chopstick, I love them) . I start stick probing when the thermal probe in the middle of the flat is about 192-193. Most of mine don't go past 195, though I've gone well over 200 on more than one brisket - they are all different, so you cannot cook to temp - but I hold them warm for a very very long time. If I weren't going to hold them warm for hours and hours, I'd generally go a bit higher in temp/softer on the probe. I like mine to have just a bit of chew or bite.
I didn't even own a digital thermometer until about 2015. I had to rely on a toothpick or a chopstick or something similar. I never used a knife blade to probe, I think a small diameter piece of wood is easier to get the feel for.
I am of the opinion that if a brisket offers no resistance on the probe, then it is overcooked. A lot of people want it to probe like it's going into a room temp stick of butter. I want mine to probe like I'm sticking a toothpick into a frosted cake. It needs to go in easily, but you need to feel just a bit of resistance. Some say to stick a toothpick into a jar of peanut butter, and to me, that is more what I'm also looking for. it goes in pretty easy, just that hint of resistance.
Your probe (the one you measure meat temp with) may be off. I'm down to 2 probes for my 4 probe Inkbird and one of them is starting to run a couple of degrees off - which stinks because I was bragging not 6 months ago how great my Inkbird probes had held up, so I'm just about to start shopping for a new thermo unit. Ice water or boiling water is the best way to see if your probe is measuring correctly. Your pit can run anywhere between 220 and 250 (those were my goals on a stickburner, and my pellet runs about a 15-20 degree swing also, and still turn out perfect meat. Radical swings are bad, 20 degrees is nothing.
