You know they made Metroid Prime Pinball before this right?
It's a spin-off, I wouldn't qualify Link's Crossbow a real Zelda game but it's still there.
On top of the done-to-death "We haven't had a new Metroid in 5 years, and that was Other M, so we've really been waiting 8 years" argument that doesn't apply to Prime Pinball, it still had a Metroid aesthetic. Not only was Samus the ball, but the environments looked detailed (Better than this game's despite the decade or so difference, honestly), there were Metroid enemies that showed up (Instead of "Ice Beasts") complete with a Meta Ridley bossfight, and various other Metroid elements like Morph Ball Bombs and the screw attack insignia were everywhere.
Spin-off or not, it honestly felt like a Metroid Prime-themed pinball game. Thanks to the change in art style for the GF troopers, the only thing we have confirming this is a Metroid game is the title and the fact that the redshirts are called Galactic Federation Troopers. Even in an ice area that could be argued as "Phendrana Drifts in a simplistic art style," it's just called a Snow Planet, and instead of the Sheegoths, we're seeing "Ice Beasts" (I still can't get over that name!).
There's a reason people thought Blast Ball was a new IP when it showed up in the NWC, with only a few people "calling it" about it being a Metroid spin-off. Samus and Metroids apparently do show up, but all we have to back that up is some developer's word, and even then, it sounded like a cameo and a single mission, respectively. I think some people are upset that this is a Metroid game, when it clearly could have just been a new IP (And at this point, they might just make it one; this backlash might be overly dramatic, but it can't be good in the long-run). These people didn't want to see the series back unless it was a Prime 4 or a Fusion follow-up. Personally, I'd've been fine with a spin-off, like a game starring Anthony Higgs, Sylux, or Ridley, but this game just looks uninspired to me.