I don’t think you understand how Intellectual property works in the context of a commercially developed game. If you are contracted to make a game based on an existing property the people who contracted you have a say in what gets made.
If Barking Dog wanted to make HW:C about how the Bentusi were a race of Blueberry muffins out to destroy the universe’s supply of chocolate chips someone in charge at Sierra/Relic would have probably told them no. Similarly if someone in charge at Sierra/Relic disagreed with making the Kushan the exiles they would have said so. The fact that they didn’t meant that someone in charge thought they were the canonical exiles or at least thought they were acceptable as the canonical exiles.
Now yes, there’s a difference between what management thinks and what the original designers thought. Not being one of the original designers I can’t speak for them. It’s just that if the general consensus at Relic was that the Taiidan were the canonical race I imagine that would have come up somewhere.

