Fixed layout epub won't zoom

Although Apple added features to Pages v10.1 that allowed it to open iBooks Author documents, it is not a replacement application at this time. If you are using a Trackpad, and its System Preferences : Scroll & Zoom : Zoom in or out feature is selected — and you still cannot use this form of zoom in a Pages generated EPUB when viewed in Books, then it likely is a feature unique to iBooks Author generated documents.

In macOS Mojave 10.14.6, I can export a Pages document to EPUB (fixed), and when opened in Books, I can press ⌘+ to expand the size of the document window, or ⌘0 (zero) to restore it. There are no other zoom controls available in Books.

I wouldn't be surprised to see another round of Pages, Numbers, and Keynote updates to reflect user interface changes in Big Sur. Only time wil tell.