Right now when installing the extension I get this warning:

The extension MJML Official is published by MJML. This is the first extension you're installing from this publisher.

MJML is not verified.

Visual Studio Code has no control over the behavior of third-party extensions, including how they manage your personal data. Proceed only if you trust the publisher.

To verify with the marketplace is simple; you only need to verify with the marketplace that you own mjml.io domain.