"Inherently" means essentially "no matter how you do it". If you use an encrypted email provider to send a message to another user on another encrypted email provider, it's perfectly secure. Ergo, it's not "inherent".

Full E2E encryption was only implemented recently by certain providers within their own domains

It definitely works across domains. All you have to do is point your domain at your preferred secure email provider.

and won’t work across the board

It doesn't need to.