I'm not sure it's technically true; the text on the page needs to be loaded, but that doesn't mean every other HTTP request associated with the page has to have

I'm not sure it's technically true; the text on the page needs to be loaded, but that doesn't mean every other HTTP request associated with the page has to have completed before Safari makes the Reader button available. While this is entirely anecdotal, I can definitely see that button appearing before pages are fully loaded.