The post you responded to addressed sending an email from one Gmail account to the same Gmail account. If you read the first paragraph [1] of the page you link, you will note that this does not require registration, so that does not appear to be the issue.
[1] We are excited to see how you plan to use schemas in email. You can start testing your own integration today. All schemas you send to yourself (from x@gmail.com to x@gmail.com) will be displayed in Google products. So go ahead and try it out now!
[1] We are excited to see how you plan to use schemas in email. You can start testing your own integration today. All schemas you send to yourself (from x@gmail.com to x@gmail.com) will be displayed in Google products. So go ahead and try it out now!