The difference is the first way to warn on the original page, before creating a new tab, and in the second way, Chrome closes the tab immediately and displays a warning on the new page.
The engineers are more interested in the first way because the second way is to allow the creation of a new tab and not to keep the history of opening the old tab, not using the Back button to return.
The third option is to improve the existing pop-up blocker and support Tab-Under.
Documented 'Intent to Implement' means that it will soon be available for developers. According to Charles Harrison engineer, Tab-Under blocking feature will support over 5 Blink platforms: Windows, Mac, Linux, Chrome OS and Android, while Android WebView does not.
When launched, it will be available in Chrome Canary under chrome: // flags. There is currently no information about the version of Chrome that will have this feature for initial testing.