Yes we can modify them. the Use Case we have is to add a new "News Post" from within a teams tab. This seems to be indicated in the blog post above, however I'm guessing that is a pre fast track release, or a rendering. :(
So, this is because the screenshot was made in the web client. It uses the current browser to render it when using web, unlike the desktop client uses some back end edge type zombie rendering engine so it trims up some of the page.
Hoping the SharePoint integration fixes this that's coming some day, but honestly the actual back end website rendering tab inside the client needs to be updated before this will work properly like the screenshot.