A few thoughts...
1. My kettle is 16 degrees hotter in the dome than at the grate. That means you were cooking around 235F, and I have seen as much as 25 degrees. You need to be certain of that variable.
2. You most definitely did not get that cooked enough. It is not done, I can tell by the texture where you cut it. Even a flat, cooked properly has a relaxed grain texture, it will pull apart as you slice it just a little, yours is still very tight, makes super clean slices, not quite there.
3. You monitored and put way too much emphasis on internal temperature, this is your second flaw in your cook (the first one is that you do not really know what your grate temperature was). The internal temperature on a flat is useless. Also, taking tips on how to cook a packer and then doing a flat will lead to problems.
4. Even a flat, cook hotter. Run your cooker at 275F grate temperature, it will be fine. Cook it until the color is a nice mahogany, probe it, most likely it will be hard. This is fine. Foil it, adding 1/4 cup of beef broth, and put it back onto the pit. Note, at this time, give the packet a squeeze, it will feel hard, remember this. After a couple of hours, stick it with your thermo, is it soft? hard? if you have a Thermapen, take a temperature. If it is not soft, note the temp. If it is 165F, you have at least another hour before checking, if it is 180F, then you should check in 30 minutes. If it is 195F, then check in 15 to 20 minutes. Also, squeeze the packet at this time, if it is soft, then you are close and should be checking every 20 minutes, if it is hard, regardless of IT, let it run for an hour. A flat that is done will have give, even under a couple of layers of foil or paper.
5. A flat will never jiggle.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"I'm Armenian...perhaps...but then again...maybe not..."