They're calling themselves a movement, which always makes me suspicious, (and that 'great diversity of persons' line didn't help much either.) Referring to themselves as marginalized suggests that they sense oppression and victimhood so they'll, no doubt, be lobbying for protected class status in the not-too-distant future.
I share your suspicion of the word "movement," especially because I often imagine the word "bowel" ahead of it. However, I think the idea itself is not a bad one.
Marginalized? Maybe. But Brights don't want pity at all, as far as I can tell. It just isn't obvious to most people how central a belief in god is to many things, and how important superstition and supernatural beliefs are to many people.
It's not oppression, but it is a sort of group-ism. "Atheism" is almost a dirty word to many people. Can you imagine an avowed atheist getting elected president? I can't in today's society. There is something wrong with that, considering that PotUS is a secular job.
No, no pity is needed. But what the Brights feel is needed is to call attention to the lack of superstitious belief as valid, and the benefits of such (not for prostelitizing, but just recognizing why someone else would be a Bright) . Looking at the goals you list in your post I'm in complete agreement with what they are saying. Especially the second one:
Gain public recognition that: persons who hold such a worldview can bring principled actions to bear on matters of civic importance.
I say "especially" because sometimes I think that people think of atheists as something like Billy Bilings in this Ruben Bolling God-Man comic. "Gosh - I can do anything and God-Man will never know!" As if there is no reason to behave with goodwill if you do not believe in a supernatural force which will punish you if you do not.
The words "movement, " "diversity," and "marginalized" (and the fact that many of them are liberals) shouldn't frighten you because of the way they have been used by others. You won't see these folks suing for reparations any time soon. I suspect they're just looking for a little social validation in what they see as a world sometimes hostile to them. And I believe you can take their goals at face value.