If the lore is subjective depending on who interprets it, how can we have a cohesive roleplay and agree on...anything?
That's the beauty of it - we don't have to agree. When you play on a server without an established canon where everyone can be who they want, you won't have cohesive roleplay. And it's okay. Establish a phase with a specified canon to fix this, and roleplay with people that share your view on things, or are able to accept it.
Sure, a faction description says they allow other survivors in - but do we see them anywhere? Are they named, mention anywhere else other than one sentence? Does it specifically mention races of Azeroth are allowed to bear the title? No, not really, it's terribly vague and I do believe Blizzard's lore writing is as cohesive as a four year old's story about going to Disneyland. It's incomplete, it lacks support, and I personally don't see it as an enough reason to say "Turalyon can tell my paladin he can join the Army of Light" - I see it as enough of a reason to see what happens in the future though, and wait for it to be fleshed out until I am given a definite answer. Until then, I will go by what has more support in the actual in-game representation.
(Also because, what other survivors could the description possibly be referring to?)
I don't entirely concern myself with "possibly" - I like to see things and read on them. To explain further.
- The army takes in survivors of the Burning Legion's assaults.
- The army happens to be consisting of draenei, 2 humans and 1 dreadlord at the present time, including the survivors.
- The survivors are there but you can never see them, so just assume they're there.
Can you, yourself, choose to believe in the second option? Absolutely, more power and opportunity to you. The argument that races of Azeroth are said survivors is, to me, too vague to take at face value, and I've yet to see it confirmed, so until it is I am going with the first one.
If the artwork released by Blizzard further supports this version of the lore by including dwarves, gnomes and the such among the ranks of the Army of the Light, then all the better. As for in-game representation, Turalyon leads a mix of Lightforged Draenei and Human Paladins in Stromgarde, it's not concrete evidence for anything (and it rarely is with anything when talking Blizzard), but I thought it was worth mentioning.
Alternatively, the artwork presents the adventurer / player character fighting alongside the Army of Light. Turalyon is still one of the original five first paladins, so no wonder he's leading paladins into battle, especially now that Army of Light is a part of the Alliance.
We can go in circles about how we perceive this case, really, but I do believe I've already said all that I had to say on the matter.