Hey Matthew, welcome to the Community.
Yes, there’s no page-level sharing — only a doc-level sharing.
This is counter-intuitive if you’re coming from Notion. This is intuitive if you’re coming from docs — e.g. Sheets. You also can’t share just a page from a Google Sheet or a page from a Microsoft Word doc. Coda is a doc, quite literally. When you load a doc, think of it as loading the whole file. Parts of it are hidden but you’re loading the whole file every time.
To share portions of data, you can use Cross-doc or my custom Cross-doc pack. The former is included within Coda at no extra cost but is somewhat clunky to setup, especially if you want to have e.g. a central doc for all meetings but then export each meeting into a separate doc that you separately share with the attendees. My pack costs a little extra but it solves this problem with row-level access rules.