That's the point! The brine is frozen at -19c and liquid at - 17c which is well below the freezing point of everything else in the freezer.
Or you can have normal water which is frozen at -1c and liquid at 1c. Your pick.
The handy thing about the brine is that its absorbs heat at a temperature below that which everything else thaws. So as the brine thaws it keeps your freezer at -18c which means everything in it is still frozen.
Of course, you need to choose a salt % that gives a melting point just above the temperature you want to keep everything else, so that the salt water is normally frozen.
I mentioned -18C because that is the recommended minimum temperature for freezers such that some foods can last almost indefinitely; certain chemical reactions are completely shut down.
Or you can have normal water which is frozen at -1c and liquid at 1c. Your pick.
The handy thing about the brine is that its absorbs heat at a temperature below that which everything else thaws. So as the brine thaws it keeps your freezer at -18c which means everything in it is still frozen.