Do plants count? Plants in the D&D setting can talk ('speak with plants') and are therefore sentient. If those races don't count under their racial definition, why are you allowed to murder plants and animals in order to consume them?
While I know many GMs and groups who love thinking about all of this, it's helpful to note that it is not required. I've been running "gray area" campaigns for decades and nobody has ever asked about the rights of plants. (Though I have had multiple animist PCs who routinely "thank the spirits" of plants and animals before harvesting or hunting them.)
When they are talking about "a specific race", what counts - orcs, beholders, mind flayers, red dragons, vampires, or glabrezu? Are all of those treated equally? Are all presumed innocent until proven guilty? If not, then why not?
The belief system will need to seem plausible and believable.
For many play styles, it also acceptable to build the world as you play. You could go through the bestiary and figure out the status of every creature, or you could just have a general idea in your mind and let the details emerge in play. All I need to know is that these people believe that orcs are irrevocably evil. I don't have to predetermine whether that is True. I can wait and see. When players ask a surprising question about the world, I often turn it back to the group to decide as a whole. "What do you all think? What would make the world more interesting and fun to explore?"