The chat help includes the line "Some links will be automatically inlined if posted on a single line by themselves, such as":
I interpreted this "oneboxing" as only having to do Shift+Enterpaste-in-urlShift+Enter. This resulted in the URL on a line by its own, but did not expand.
What I needed to do was actually press Enter before and after and provide the link in a post sent on its own. Could the help be reworded to make this less ambiguous?