All right, I bought the book from a German store now. It indeed says it's 3.0 in the content.opf, although there is nothing specifically epub3 to it as far as I can tell. It even does have a toc.ncx, if that is a criterion. There's some weird nav.html in place of a regular inline-toc. But there's definitely no multimedia content or anything.
So it seems Kobo really make it kepub-only as soon as the book says it's epub 3.0...
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