Their argument is that the Voice isn’t even something good. It doesn’t give Indigenous people any powers they didn’t already have, and the Voice can be ignored just as easily as the advice of the royal commission into Aboriginal deaths in custody recently was. Interview with the Black Peoples Union describes in better detail.
But even if that weren’t the case and they did think it wasn’t worthless symbolism, successful collective bargaining doesn’t just settle for every first offer. So I don’t know why you’re claiming it’s a bad strategy, it’s how unions have won important gains for workers. It’s a strategy that has been historically shown to work when applied correctly.