My Devil's Advocate^TM question, however, is: why exactly is this person, or this group of people, the spokesperson for their entire race, or all PoC's to you?

She's not the spokesperson for all PoCs to me, by any stretch of the imagination. PoCs are not members of the same community simply because they're PoCs, and to treat them as such would defeat everything everyone has worked for.

She is, however, the spokesperson of her community to me because that is her hired role. She was sent out by her community to try to change cultural representations of her group and to educate people on how her group has been wrongly stereotyped (and, as such, injured by said stereotypes), so when she tells me that her community holds the views I described above, I have to accept that. One of her community given directives is to get outsiders to stop representing them since they want to be allowed to speak with their own voice, and that apparently means to start by silencing the voices that already exist.

Since, in the absence of other information, I try to respect the desires of the community, knowing that one such as my colleague's exists makes me very wary of all the other ones that I don't know much about--which practically be definition, is nearly all of them out there.