I sent the breeder an email...
I read today that they were used a LGDs back in the 1800s.
https://retrieverman.net/2012/04/16/the-original-function-of-neapolitan-mastiffs/
My source for this bit of information comes from the 1908 Diplomatic and Consular Reports from the British Foreign Office:
The Neapolitan mastiff is a heavy and useful cattle dog. He has a fine smooth coat and is generally brindled or black. He is kind by nature, but easily made savage if he is tied up or ill-treated. His original use was to go out with the cattle and goats and protect them from the wolves which in olden days were very common all over the district. Then, as now, he guarded the homestead at night (pg. 15).