Use iBooks Author to generate works for iBooks use another tool to generate works for other media. IANAL but I don't think they're saying that if you use iBook

Use iBooks Author to generate works for iBooks use another tool to generate works for other media. IANAL but I don't think they're saying that if you use iBooks Author to make your work available for iBooks and use amazon's tools to make your work available for Kindle that you'd owe Apple a cut of your Kindle revenue.