Oh, come on, Dagon. They root in Argonia, they represent the People of the Root. Yes, there are rogues, there were rogue Ehlnofey, but there's hardly a distinction between what they are. They're telepaths.
There's a difference between not having individuality like we think of it and being able to conflate what, where and why they are with each other. From what we have so far, whatever mythic significance the Lizard-folk wield, they gain from the Hist. Yes, you can argue that they become more Hist-like when they lick the sap (though I wonder what makes the poison that that one Hist produced for the Blackwood Company), but there remains a difference between Tree and Lizard. From there, we can pretty easily find that there is a difference between Lizard and Argonia and Tree and Argonia.
Nitpicky and unrelated point: It's more accurate to say the People of the Root represent the Hist (hence "of the root"). Though I suppose the difference is negligible in the end.
You are aware, Mehrunes Dagon, that the "Argonians" are already known to call themselves the Hist? The distinction between the lizards and the trees is entirely imposed upon them.
If I'm remembering correctly, that comes from Redguard and was never expanded upon and while it's not been contradicted since, one would think we would've gotten some hint of that after getting into a somewhat mythically significant Argonian's head for a while. So I'm calling it obsolete. Yes it is convenient for me to do that, shut up.
(Also PGE1 ain't the most reliable source about these sorts of things.)