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What you're trying to open is not technically even an epub. It's just been given an .epub extension. The opf file in that kind of archive is for metadata purposes only. Sigil's not going to accommodate any-old zip archive full of html/css/images just because the extension claims it's an .epub. It must actually BE an epub (or it will be forced to be). And being an epub means certain strictures have to apply.
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