What you were seeing was a topic featured link, not a tag. I can reproduce it by pasting the link https://en.wikipedia.org/wiki/Death_by_coconut into the composer’s title input on my site. For it to work, the topic featured link enabled site setting needs to be enabled.
Possibly the user created the topic by pasting the link into the title input, and then edited the topic to update its title and remove the Wikipedia onebox. Can you see any edit history for the topic?