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.
Ultimately, it's important to remember that BlueSky is a for-profit business, like Twitter, like reddit. I urge everyone to avoid it where possible, just like I would go back in time and urge people not to make Twitter a thing.
They will inevitably go down a similar path. Even in the best case hypothetical scenario, they are still beholden to the interests of shareholders and advertisers. They have to make money from you, or from rich companies, to survive. Mastodon instances, on the other hand, are scalable enough that they can sustain themselves off self-funding or donations. Just like Lemmy, they don't have an intrinsic motivation to throw in ads, or to get you addicted to scrolling and arguing, or to censor communities that offend their sponsors.
It's no co-incidence that you're feeling some similarities between Lemmy and Mastodon, in fact Mastodon users can actually post here! 'Fediverse' programs all use the same language (protocol) to communicate and so some are able to interact. I've had a Lemmy<->Mastodon conversation before. Admittedly it's not ideal to do that everyday, because of the obvious difference in formats, but having the ability to do that can be useful, especially if one service has a community that yours doesn't.