I haven't seen this episode, but I'd be surprised if anyone on the crew believed that this artifact was Roman, including the person that identified it as such. A quick search on the internet would have provided a likelier answer. The fine folks on this forum have convinced me that it's from a gig of some sort, and I didn't even know what in the blue hell a gig was until I read this thread.
Now put yourselves in the shoes of the people making this television show. Do you tell the audience that this is part of a frog gig that was commonly used in that part of the world, or do you have an "expert" excitedly proclaim that it's a Roman spear, which was never used in that part of the world? Which path produces the sort of television that's more interesting and exciting for the average viewer? Your job is to convince them to watch your show, not to educate them, and not to be honest.
I don't blame the people that make these TV showers for being dishonest, just as I don't blame the producers of movies, novels, and other works of fiction for saying misleading and/or downright false things. They're entertainers that are selling a product that the public wants. I blame the dupes that believe everything that they see on TV - i.e., the public - for not thinking critically and eating this garbage up.