"Whether or not Altman wanted SJ's voice" and "whether or not they got someone else to do SJ's voice before asking SJ to do it" are two completely independent matters.
What does 'after' mean to you? Strange response.
Do you think they only got one person to do it or something?
If they weren't looking at multiple options then ... why did they still ask SJ?
Oh right because they asked SJ because they hadn't picked yet.