The issue is not whether the WiFi is on, the issue is whether something is using the WiFi. I rarely turn WiFi off on any of my Kobo devices. The only reason I do is for some sort of testing, or when I know there is an update and I don't want to do it right now.
By the sounds of it, the sync, including downloading the book, took longer than usual. I suspect that you rarely do a sync, so this one would have taken longer. It would not be typical, so it wouldn't be good to judge the battery life on it. Try it again and see what happens.
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