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If specifying line-height, extra spacing between text blocks and any padding around images should be a multiple of the specified line-height to keep text aligned across the spine.
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That is a truly strange strange requirement. I have not seen anything like it for other e-book platforms.
You mention Kobo and ADE. Some Kindle apps and devices can also show side-by-side pages when in portrait orientation. I have read the Kindle Publishing Guidelines and there is nothing like this Apple page alignment requirement there. Perhaps someone more familiar with Kobo can comment about that platform's requirements.
The requirement itself is also odd and subject to different interpretations. If might be seen as applying only if the publisher wishes to to keep text aligned across the spine. Or only if a line-height is specified in the document. It uses "should" instead of "must" which usually indicates something is suggested but not required.
Finally doing what it says won't even work in many cases. For example the presence of images on the page will throw the placement of text that follows out of alignment unless images are also sized in multiples of the line height.
I find it hard to believe that books from commercial publishers are modified to follow this rule, but I could be wrong. It might be a good idea to ask for clarification in a forum specific to publishing for Apple.