@JuzamJim You're actually wrong on both counts.
All the last sentence says is if you don't cast the revealed card, put it into your hand. So you can put any card you reveal off of this into your hand. Even if you play a mox and reveal a gush you can't cast the gush for free, but you put it into your hand. It only cares about the CMC of the card if you're trying to cast it.
It says you can cast it without paying its mana cost, this is similar to miracles or cascade were the ability lets you cast it right at that moment even if normal timing would prevent it. So just like in legacy where you can cast a sorcery terminus in your opponents combat step, or when you cascade into tarmogoyf and can cast it even though there is a shardless agent on the stack, you can cast a monastery mentor at end of your opponents turn if you reveal this after you cast a dig through time.