I had the very same question as I feel like being tricked out as the whole game, including the EE would have been never be possible.
If Larian wants to keep their credits manageable then they could add a button for their short internal credits and one for backers, maybe even with a simple sort/seach function by name because the credits will be loooong with the backers and not everybody might like a marathon session like in the Lord of the Rings movie credits.
But still, I hope Larian will add the backers back into the credits as (almost) nobody will touch the original game anymore with the new edition around.
For the amount many put into supporting their games I think that's not too much to ask for.
Sure, for now they will be busy patching and clearing up more pressing issues but I hope we will get a response that they will eventually fix the credits at some point.
Thank you in advance.