I'm taking the liberty of adding a little more to the story about the cross and the statements made by the descendants of the three discoverers of the well.
1/ These poor ladies are simply repeating what they were told... and it's quite interesting to note that, privately (unofficially), the story they heard was one of success — the discovery of a treasure — rather than failure — finding nothing.
2/ It's actually pretty clever to publicly (officially) claim that no treasure was found, in order to sell the island at the best possible price. After all, the buyer would be purchasing the potential of a treasure rather than just some land, trees, and a swamp.
3/ It's very likely that the treasure was divided up by the original discoverers, and the McGinnis family kept the cross as a kind of concrete proof of the unofficial family legend of their success. This seems fairly plausible if you're willing to believe that a treasure was actually buried on Oak Island.
4/ The key piece of information shared by the McGinnis descendants is actually disastrous for the Laginas, because it completely destroys the core appeal of the TV show. They’re basically saying there’s nothing left to find — the treasure was already found — and they have in their possession something the Laginas haven’t been able to find in 15 years: a gold artifact.
5/ If there were any real doubts about the authenticity of the cross, I believe the Laginas would have every reason to make it known, because it would cast doubt on the old ladies' revelation. Interestingly, aside from two or three episodes, the show never mentions the cross or the claim that the treasure was already found ever again.
6/ As for the possible origin of the treasure... a few episodes briefly reference the expedition of the Duke of Anville and his logbook, which is kept in a museum and seems to be authentic. It mentions French soldiers' pay being buried on an island full of oak trees in Mahone Bay... Yet again, a highly specific and apparently credible piece of information that's brushed aside to keep spinning endless theories about Freemasons, Portuguese explorers, the Templars, and so on.
7/ The TV show's goal is to keep going, not to reveal the truth. But I think the truth has already been given: if there really was a treasure, it was French, and McGinnis and his friends found it. Afterward, they spun a whole story to sell their island for the best price. The tragedies that followed prevented the truth from ever coming out, out of fear of lawsuits or revenge. I believe the McGinnis descendants are fully aware of those risks, which is why they prefer to keep some doubt alive about their story and the origin of the cross.