"Relevant skills" are the things listed as "required" or "preferred" in the job ad. These typically are all of those things.
I agree with your second paragraph--but isn't that a strong case for investing in a cheaper developer with less experience?
When I started, I was expected to pick anything up as I was working. I got my first job partially as a result of having seen Smalltalk in a programming language class. The important "relevant skills" were enough background to pick up new things (since everywhere was different) and a demonstrated ability to get work done.