I think by "invisible message" they mean it's invisible to the receiver because it's in their spam folder. I also thought this was a way to send a normal email to the spam folder, but looks like it is assuming the spam filter removes the email based on content. I wonder if there is a way to use this method with a tracker pixel, so that it gets filtered out because of the content, but the reader can't see it.

edit: I tried it in gmail and it was invisible.