How do you suggest it got into a nuclear emulsion? It’s not the same that a hard surface and the process ensures it doesn’t get scratched.
These scratches are easily made on photographs. Simply sliding photos, one on another, is sufficient.
This material was fairly soft, so a thick imprint was made of the physical particle that made it.
If it was actually inside the emulsion that would be from something different.
Normally atomic-type particles expose the emulsion to make a track. That doesn’t look exposed to me.