Let's look closely at all major muscle groups statement. Is it a truth or a lie? To help us bust this myth or not, I found the following video which gives us insight into what muscles are actually being used during swimming of all the strokes. The video is a work of Dr. Nakashima from the Tokyo Institute of Technology and I am not sure anything else needs to be said, just watch and you will form your own opinion.
So, to answer the question, "Can dryland exercises help me improve my swimming?" we would have to state that it is a BIG YES, so get out there and start doing something.