You can; the issue is that you can’t add two snap repositories at once.
This is functionally pretty much the same thing, as nobody is likely to want to use snap while locking themselves out of the main snap repository, but it’s still important to make the distinction.
In theory I guess there’s nothing stopping you setting up a mirror of the main snap repo with automatic package scraping, but nobody’s really bothered exploring it seeing as no distro other than Ubuntu has taken any interest in running snap.