It's true, but for most cases, the volume of a fiber is not the problem anyway. Latency is a problem most of us somehow bump into every day, while most fibers in the ground are nowhere near what you can push out of DWDM (e.g., off-the-shelf equipment will easily allow you to run 20x100Gbit over a single fibre, but many of them only carry a single 10Gbit or even 1Gbit link).

Trans-continental is different, because you'll need amplifiers. Many, many amplifiers in a row. And those generally work well only in a fairly limited band. But unless you're doing submarine, bandwidth is almost never the problem.

To make things worse, a lot of existing medium-haul fiber links are actually twice as long as you'd expect, due to the desire to cancel out dispersion; you first run the fiber e.g. 10km from place to place, and then run it through a large 10km spool (of a slightly different type of fiber) in the datacenter to cancel out the dispersion. This is slowly going away, but only slowly.