The data the linker is running on will generally largely be in memory (in their posted example, which is a warm compile, completely in memory).

Yes, but that's for mold, which is multithreaded. The original context of this thread being the question of whether a linker would see speedups from multithreading. Most people are using traditional single-threaded linkers which are an order of magnitude slower than mold. The fact that mold is so much faster suggests that a linker does indeed see big speedups from multithreading.