Here is the deal with flats.
First I will speak generally about brisket. If you EVER get a brisket that is no tender then you didn't cook it enough. Briskets cooked enough, dry or otherwise (unless you cold smoked the damn thing) will at least fall apart. So the fact you said it was dry AND not tender tells me you did not cook it enough.
Now about the flat. Oh hell... actually back to the brisket...
It is a complex and weird cut in that it will look half way or more into the cook like you dried it out. Wierd thing is... you did. Most of the water is gone from it.... oh but there is the other stuff, the honey that is in there waiting to be released. This comes with time and heat.
Case in point... look at this video...
and then this one
Same brisket... but the first video was at 9 hours.... after that it went loosely wrapped in paper and foil (I used foil cuz I had no diffuser so it might burn) and added NUTHIN!!!!
As you can see in the second video the flat... which was so tough it hardly probed, was actually done so much it fell apart. I think you can see the juice.
Flats, if I do them, frankly I let myself off the hook in the ego department. I smoke slow to set the ring, crank it up and rush to the stall, then wrap and slow way down
to get it all juicy.
Hope this helps...