There's no discussion to be had if you're not willing to budge on this. Long standing lore in second and third edition codexes has been retconned in more recent editions, stories told in codexes are sometimes contradicted by novels, and novels occasionally completely get the lore wrong. To claim something is canon simply because it existed in the first edition is folly, especially considering it has never been mentioned since.
Trying to pin specific things down as canon gets harder the further back the lore goes, especially given the amount of things that have changed since then. Just as a few examples, there were no mentions of Perpetuals in earlier editions, and the lore regarding Living Saints was much less fleshed out. The role the necrons had in the wider setting has been thrown into question, and the very nature of the Warp itself has been altered with the introduction of the material called Blackstone just this edition.
The Emperor could simply be a powerful psyker and a perpetual for all we know. He could be using Chaos Himself. He could be manipulating Blackstone somehow. There simply isn't enough confirmed anymore to say with absolute clarity that the Shaman Origin is canon, even if it is still cited on pages like Lexicanum.
And given that many younger/newer players aren't even aware of the older lore, trying to assert it is more or less pointless.
The shaman origin is not just old, its from the lore of 1st Edition Rogue Trader. I'm willing to bet Endymion was a younger player.
And I gave permitted links to the book with the shaman origin in my thread post here.
Last edited by Justcause12345#; 10-16-18 at 01:42 AM.