It's been a long time since college biology so correct me if I'm wrong, but for all of the races to be the same species, wouldn't they have to be able to all produce viable offspring with one another? We've seen examples of NPCs that are half Sun Seeker and half Moon Keeper, but until we see an NPC who is, say, half Hyur Midlander and half Elezen Duskwight, or at least hear about such a case, I wouldn't be able follow the same line of thinking.
More on-topic, I think it's entirely possible that the level of "cat-ness" of a Miqo'te is something that's on an individual level, not a species/race/clan/tribal one. The NPC that says "meow" instead of "now" is very obviously a Moon Keeper but she is, as far as I know, the only Miqo'te and the only Keeper to speak in that specific way. Maybe it's something common to the Molkot family, but I don't think we can say "All Miqo'te speak/act/feel like so-and-so."
More on-topic, I think it's entirely possible that the level of "cat-ness" of a Miqo'te is something that's on an individual level, not a species/race/clan/tribal one. The NPC that says "meow" instead of "now" is very obviously a Moon Keeper but she is, as far as I know, the only Miqo'te and the only Keeper to speak in that specific way. Maybe it's something common to the Molkot family, but I don't think we can say "All Miqo'te speak/act/feel like so-and-so."