Despite Stan's testimony, the insistence that all (or nearly all) ropes on shipboard be called "lines" is, so far as I know, an arbitrary imposition by disciplinarians trying to instill seamanship in raw recruits in the marines as well as the navy. Trainees in both branches (at least in the United States) are indoctrinated with a number of such usages.
Any floor and even the ground must be called "the deck." Any stairway must be called a "ladder." Any hat or cap is a "cover." Any kitchen is a "galley". Any lavatory is a "head." And so on.
If you've experienced such training, it's easy to become doctrinaire yourself. But plenty of earlier sailing ship writers, including Joseph Conrad, Richard Henry Dana, Jack London, et al., used "rope" in their works.
Reminds me of some remarks by Conrad that only idiots, and not real sailors, spoke of "casting anchor." He insisted the proper term was "dropping anchor" or just plain "anchoring," since, except in the smallest boats, anchors are not literally "cast" or thrown anywhere.
The record shows, however, that in the generations before Conrad - and since - real shipmasters and other authorities frequently spoke of "casting anchor."
Another of Stan's sayings was, "Different ships, different long splices." In other words, it all depends.