This works for errors at the origin or in a Worker, but won't for errors at the edge, because those are in front of Workers.

I think Gemini refers to Web Workers API rather than a Cloudflare Worker. And that should work indeed.

the custom error page is configurable at a domain (zone) level

which sometimes gets annoying because branding for subdomains could be different.

https://developers.cloudflare.com/rules/custom-errors/edit-e...

> Error Pages do not apply to responses with an HTTP status code of 500, 501, 503, or 505. These exceptions help avoid issues with specific API endpoints and other web applications. You can still customize responses for these status codes using Custom Error Rules.

From that page ;)