Brining...at high temp?
Brining works at a cellular level; if the solution does not have the time to act and reach inter-cellular gaps then you won't get the benefits. 

How does brining work (from same wiki article):


the meat cells have water and solutes (sodium, chloride, potassium, amino acids, ...etc)
the brine has salt (sodium, chloride)
the salt ions of the brine diffuse into the cells
the osmosis cause the cell to absorb water
some other stuff happens to the proteins (denaturalization) causing the water to be trapped


This is a slow process, in particular for the salt ions to diffuse into theat, for the osmosis to happen, for the protein to coagulate ...etc.

How is brining different from curing:


the concentration of solutes (e.g. sodium and chloride) is different
curing is usually mostly done with salt (sugar/molasses/...etc optional)
the concentration being higher, the osmotic pressure draws water out.


Slow cooking in the brine just didn't give a chance for the meat to absorb and trap water. Since you describe the meat as tough and dry, I would say this is mostly because of salt concentration seems too high (wet-curing).