Well, I don't see an opus number on the score, whatever that means.

Opus means work. Opus 34, e.g., on a piece of music simply means that that piece of music is the 34th piece written by that particular composer. An opera (plural of opus) is so called because it is a collection of works.

The joke is that Verdi was best known for his operas and the music of the Requiem has a very operatic feel to it. There were other aspects of it that the speaker of the workshop mentioned that were operatic in nature but to call it an opera was to ignore its true nature.

It grew out of a single section that Verdi had written for a project that he had started when the Italian poet and novelist Alessandro Manzoni died. Verdi suggested that the great composers of Italy collaborate to compose a Requiem to honor Manzoni and he took on the task of writing the Libera me section. The resulting Requiem was a failure. It was never performed but Verdi built his Requiem on his section. That Requiem was not a failure.

Verdi, by the way, was an agnostic.