No, you're excluding the set of emails that are entered incorrectly and thus are not valid. The result for those is not the same as if the UI included a simple test (such as your "ambitious" example).
1) Without UI validation:
- 1.1) Email address entered correctly -> activation email sent
- 1.2) Email address entered incorrectly but forms a valid address -> activation email sent to wrong address
- 1.3) Email address entered incorrectly but doesn't form a valid address -> activation email not sent
2) With UI validation:
- 2.1) Email address entered correctly -> activation email sent
- 2.2) Email address entered incorrectly but forms a valid address -> activation email sent to wrong address
- 2.3) Email address entered incorrectly but doesn't form a valid address -> user warned
-- 2.3.1) Email address re-entered correctly -> activation email sent
-- 2.3.2) other states
In the 1.3 case all of the activation emails fail to be sent. In the 2.3.1 case activation emails are sent that otherwise wouldn't be.
2.1a) Email address entered correctly -> validation fails, no mail sent
This prevents activation mails that would be sent without validation (or validation against a regex like [^@]+@[^@]+).