This is undocumented magic and it also doesn’t work. This seems like the most common use case for a website (using a real domain) and there’s no way to do it with Cloudflare Workers? Really?
It can seem confusing, but I have 3 different websites up through Workers just fine. I use an A record with whatever domain my site is, say example.com and point it to 1.2.3.4 making sure they proxy status is set to proxied.