It means don't stick a frozen bird in a brine. the variations of thawing, etc of a bird in a brine are just to varied to insure good results. Frozen is okay, just thaw before you put in a brine.
If you guys dont have your turkeys yet, it's probably too late. Take 2 to 3 days in a fridge to thaw, 2 days for brining...the clock is ticking...tick tock