If a filevault user is logged in when the computer is stolen and power is cut off (or if it is stolen some time after sudden power loss while the user was logged in), the contents of their home folder will be inaccessible to the thief because of filevault. However, with sudden power loss, the tmp file will not have been deleted, and being outside of the filevault image, it will be freely accessible. This would only affect tmp files for attachments created during that session, but it's still an issue.

But even with a normal log out or shutdown, the tmp files are not erased securely (why would you think they were anyway?), so unencrypted pieces of any attachment dragged and dropped months in the past could still be floating around on the hard drive of a habitual drag and dropping filevault user, recoverable by any thief with the right tools. This danger was already mentioned in the hint. Performing an "Erase free space" operation would probably clear that up, but it might not have occurred to anyone that doing that would even be necessary without this hint.

If the problem doesn't affect you, fine (it doesn't affect me either), but don't downplay the issue. I'd say this is a good hint since it provides the service of making people that might be affected aware of the problem, and even provides a workaround (use "Save") to avoid the creation of the insecure tmp file in the first place.