> Statistically speaking, does extremely unlikely mean impossible?

No, it means extremely unlikely. Collisions can occur, as op just found out, but the chances are so abysmally small that most people don't care.

Any application I have worked on, I always had a pre-save check to see if the UUID was already present and generate a new one if it was. Don't think it ever triggered unless a bug was introduced somewhere but good practice anyway.