You are correct. The tabs add bromide, but very slowly. I assume the tabs add the appropriate amount of chlorine for the amount of bromide they dispense. This may, or may not, be enough sanitizer to deal with the bather load.
New water should have 30ppm of bromide added to it. Then you can add an appropriate amount of chlorine to "activate" enough bromine for the bather load.
I buy a "Startup Pack" of bromide for @ $3.00 when replacing my water.
Then I add bleach to maintain sanitation and shock levels of bromine as needed.
I use tabs in a floater as a supplemental dispenser in case I don't have a chance to test my water for a few days. It's mostly a "backup" to prevent my bromine level from ever reaching zero. I don't need the bromide that it's dispensing. That's already in the water. I'm just using it to dispense the chlorine that activates the bromine. The extra bromide added is not a concern.