As you can see, the statement in that article about the Cedric is that due to its 9 watertight compartments, it was "practically unsinkable". That word 'practically' makes the phrase just a figure of speech to denote how safe the ship was supposed to be in comparison with its predecessors and most contemporaries.
For example, we all use the phrase "it is practically next door" to denote something that is quite close. It might be 20 miles away from the point of reference but that distance might be considered very close if one is looking at the bigger picture of that situation.
What would be interesting to know if the bulkheads were capped off to prevent the water over spilling from bulkhead to the next. Would that of stopped the sinking of the ship? In other words the weight of water against buoyancy.
According to Milton Watson in his book Flagships of the Line (page 98), the term “practically unsinkable” was applied at a much earlier date, to the Augusta Victoria of 1889. HAPAG, the owner, printed this term (presumably in German) in a brochure where they stressed the safety features, such as the double bottom and the redundancy built into the engines, of this ship. The Augusta Victoria was the first of a new class of luxury express Atlantic liners, conceived by HAPAG director Albert Ballin.
This suggests that the claim “practically unsinkable” might well have been a common advertising ploy intended to reassure prospective passengers already two decades before the Titanic, and that when The Shipbuilder later described the new Olympic class in the same terms, that journal was merely borrowing an old cliché.
I wonder whether anybody knows of any other instances – obviously pre-1912 – where a ship was described in this way?
Incidentally, not only the Augusta Victoria but also her three sisters – Columbia, Normannia (later L’Aquitaine) and Fürst Bismarck – enjoyed successful fifteen-year careers on the North Atlantic, and ended their days peacefully at the scrapyard.