> Quickly preview your design on real devices or share it with your team by sending it as an email message.

This refers to the ability to send the design you are working on as an email message. When I wrote it I didn't even consider that it might be interpreted to state that we offer device testing. We don't. You can choose to send to a real device, or Litmus, or a colleague. But we don't offer testing on real devices.

Still Mail Studio takes a lot of care to generate code that works cross-device so if you stick to visual editing (without writing custom HTML or CSS), your designs should work well everywhere even if you forgo real device testing.