To me, the problem (in part) seems to be that the recruiter is not qualified to understand, or at least cannot clearly communicate, whether they actually need someone who has worked specifically with copper or whether they want general experience with pipes such as copper and pex.
Recuiter: we need someone who can work with copper and pex pipes. Your resume says that you work with PEX
You: I can do plumbing. I have connected PEX to copper before.
Recruiter: Do you have experience installing copper?
You: Building codes allow you to substitute PEX for copper pipe. I have used PEX with copper.
Recruiter: we need someone to work with copper. Thank you.