I believe a step was left out though from the March 17, 2021 solution: at the end of the stated process, after the original homepage (now with a new slug) is reset as the homepage, you can revert to its original slug, thus giving your newly created redirect rule a page to redirect. That step allows you to override the "you can't redirect a page that already exists" rule which was essentially the problem.
So it's as if once a URL redirect in Squarespace takes hold, it will stay working. It's just not able to "take hold" — even though Squarespace allows you to add the rule to the URL Mappings list — if the page you're trying to redirect already exists. In other words, "/xyz" can be mapped to "/" as long as when such a rule is added to the URL Mappings list, "/xyz" doesn't exist. But if a page with slug "xyz" is created after the mapping exists (as in this case), the redirect will stay working.