Well, in this instance remote code actually means remote. In V3 one can no longer use the tabs.executeScript API [1] in which you could pass an arbitrary server rendered string.
V3 kills Userscripts (tampermonkey/violentmonkey). You will no longer be able to execute your own code written with your own hands&brain and stored on your own hard drive.
[1] https://developer.chrome.com/docs/extensions/reference/tabs/...