Email for subdomains without main domain

With the launch (https://blog.cloudflare.com/email-routing-subdomains) of subdomain support for email routing, I was wondering if one can use email routing only for subdomains and not for the main domain.

On the dashboard, it says subdomain routing won’t be enabled if the main domain routing is disabled (see screenshot below). However, I set it up, and it seems to work, so I’m curious if I can rely on this in production.