This is false.
Not only Tab doesn't automatically send the email but Google has not overwrittern anything. This is the default behaviour on a web page. Pressing Tab selects the next interactive element on a web page (a link, form elemnts etc.) and pressing space/enter activates it.
I'm guessing you press tab expecting to indent something, write a word and then press space to begin the next one which actually sends the message.
This is how forms on a webpage work and AFAIR this is how the compose function in gmail always worked. I think i have seen a couple of WYSIWYG text editors that "capture" the tab keypress and actually ident the text but this is far from the norm.
