I don't know why we assume Disney was ever in on Potter.
It isn't so much an assumption as it is a commonly accepted claim from various insiders. As the story goes, Disney was actively involved in negotiations for Potter but, unlike Universal, they refused to accept Rowling's input or give her final say on everything like she wanted, which lead to her turning them down. Disney was supposedly the first to come calling, so they in essence lost the rights to Universal.
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