It is entirely traditional. Go and find out the meaning of a catafalque. If a coffin-like box is not available, you can even replace it with just a black funeral pall.
I guess I have just never seen it... I did look up the meaning of catafalque earlier but couldn't find any good sources. 😅
My sister says they had empty caskets at their Indult Requiems at her college for All Souls Day as well...
But I don't ever remember us doing that at Saint Athanasius for All Souls Day. The only time I remember there being a casket was when there was an actual body or at least it was someone's primary requiem Mass and the body was not able to be found (like if a body was burned in a fire or something like that).