Second, they work alongside actual aliens. Being different is not a disqualification. And Maddox isn't callous, he just doesn't regard Data as anything more than "just a machine". A position he eventually changes over the series as he becomes one of Data's friends.
Data is also not a stand in for the neurodivergent. He's the flip of Spock. Spock asks us what if we tried to approach every question from a place of pure logic and repressed all emotion. Data asks us what if we didn't have the option, that we had to approach everything from logic and couldn't even feel emotion. I also feel that equating data to someone who is neurodivergent is kind of insulting as neurodivergent people do have feelings and emotions.
But Data was capable of being fully autonomous and could act with agency. Something a rock can't. Data exhibits characteristics we generally accept as conscious. He is not only capable of accessing a large corpus of knowledge, but he is capable of building upon that corpus and generate new information.
Ultimately, we cannot prove a rock is not conscious. But, as far as we are able to discern, a rock cannot express a desire. That's the difference. Data expressed a desire. The case was whether or not Starfleet had to respect that desire.