Most movies today are not shot on actual film, but are instead shot using digital cameras. Like some other directors, Christopher Nolan prefers shooting on film because he likes the look of it better than digital.
The article you've linked to is noting that, if a digital camera were submerged, all of the data it had recorded would likely be destroyed. This is true of just about any digital device that is not specifically waterproofed. Consider, for example, what might happen to any photos you have recorded on your iPhone if you were to drop it in the ocean.
Physical film does not share this vulnerability to moisture. In fact, developing physical film actually requires getting it wet. Although film isn't supposed to be exposed to salt water, the processing lab Nolan used was able to salvage it using a technique that is not specified in this article. This kind of technique would not work on any kind of electronic storage device, like a memory card.