Brining makes meat tender because the salt (and some sugar) breaks down proteins. Shouldn't a salty/sugary marinade do the same thing?
Brining does not technically "break down" proteins in meat. At least not the way marinating does.
Brining actually involves osmosis which carries salt and sugar inside the cell walls. This denatures the proteins causing them to unravel and interact with one another. This forms a matrix which traps moisture in the meat.
Marinating on the other hand actually does "break down" the proteins using acidity. The acid literally does consume the proteins and break down the texture of the meat. If the marinade has salt in it, then osmosis will occur as well and the marinade is also serving as a brine.
So, to answer your question: Yes, a salty marinade will brine your meat, but the protein breakdown that occurs is due to the acidity of the marinade.
The point of brining is not tenderization- It's juiciness and flavor.
When brining, initially dissolved substances inside the meat cells leave into the brine. When the solution reaches equilibrium they start to move back and forth, in and out of the meat. Any flavors in the brine move into the meat as well. The salt denatures enough protein to lock some of the water inside the meat causing more water to get pulled in. After a little while the meat is super charged with water that won't even cook out.
In order to work, the solution has to have relatively a lot of salt in it. But even then, the salt is not able to break up the collagen fibers that make meat tough- it takes heat and time for that to happen.
In order to get the super-charged-juiciness with a marinade it would have to have a lot of salt and water- more than marinades usually do. And then you would have made yourself a brine.