It was clarified that the information is always secret until the DM decides to reveal it. In other words, nothing they do will successfully tell them whether the treasure is actually there without going there. It wasn't in the initial prompt but he clarified it in one of the replies.
If the DM knows when, where, and why the false map was created before the players engage, its fine. If the DM does not know, that means that the map had no other purpose than to force the players onto the island. They would have ignored it if the DM gave any indication it wasn't trustworthy, but the DM locked out any signs that the map was false from player view, even when they went out of their way to find it. Their choices didn't matter, they just chose the only option to investigate further: to find it.
I use the term "McGuffin" alot as well, but I never use them in my games. There are no required item that the players must obtain to prevent them from dying, or to succeed at any one goal. The existence of a McGuffin is railroading.
Rather, a helpful item is at a location and obtaining that item may make surviving or completing the next goal easier but its never required.