I just asked my father about this and he has only a little recollection about it. He said that it was either Mosconi's wife who told him, or it was someone else who knew Mosconi's wife, but the information came from her. She said that archival material was in the possession of Brunswick and the material was kept in a warehouse that burned down, destroying everything.
Again, my father was a little fuzzy about how he came across the information, but is was a personal communication. The internet did not exist back then. He was 100% sure that this is the story he was told.
I'd be interested to hear if there is more detail on the incident. I'm sure Brunswick must know something about it.
He's right, they did have a fire....and a flood, and a fire, and another fire, and more records purging incidents than Hillary Clinton and Richard Nixon combined lol
However, I suspect that very little of those losses were actual films and/or video, because it just wasn't in their heads to record such things. Brunswick with all their greatness, may very well go down in history as the worst record keepers in history. Some of it by accident, and some of it was very intentional; they did and do have things to hide.
That said, also keep in mind the technology that was available during Willie's reign. Actual film was expensive, noisy and tedious at best, television was still in its infancy, and video archivists didn't exist...basically three strikes against future fans of the game ever seeing footage of that era.
However, as luck would have it, some footage has survived, it's just few and far between.