I haven't used it myself but it sounds like it gives the TV screen you are casting to the "player" view (not what is actually on the laptop screen) meaning it can't see through the fog of war until it is revealed.
If not you can log into the room as a player (you can do it on the same computer, you just need to use a different browser or an "incognito" window in Chrome so OR thinks you are a different user). You can then cast the player version to the TV. This is what the guy in the youtube video did.
If you are testing it out you might want to join the room as a player anyway, just so you can see how the fog of war looks from the player side.
