It was cocky and overconfident to call the Titanic "unsinkable" but one thing that's overlooked is that she was genuinely really, unusually solid. She could float even with 4 compartments fully flooded, which even a lot of modern day ships can't do.
And it's not like they were wrong about her being solid! Olympic, her identical sister ship, survived being torpedoed and then running over the U-Boat that fired that torpedo. Those ships were solid.
It's very clear that absolutely no other ship in 1912 would have been able to survive that collision, and it's a testament to the quality of the ship that she didn't sink in a few minutes Empress of Ireland style. Part of what makes the Titanic such a tragic story is that it isn't a group of rich idiots locking themselves in a shoddy iron barrel to go 4km underwater. It was 2200 people, most of whom were poor immigrants, on a reliable ship on a commonly-made journey, and then something went horribly, unpredictably wrong.
