Earlier quoted context omitted.
The default behavior for "Tab" on the web is to move to the next element on the Tab order.
The default behavior for Tab in rich text editors has been to insert a tabstop for quite some time. I think this is just a casualty of goog.editor.Editor's unconventional default behavior, which is to tab out of the field. If that is truly an oversight (not overriding this behavior), it says a lot about how the Gmail project is being run these days, because previously they did override this.
But old Gmail did override this.