Carbon repair shops are becoming much more prevalent. I'd suggest searching your local area for one so they can take a look. If there's no one within driving distance you can probably send pics to one of the better know companies such as Calfee Design, Ruckus Composites, Appleman, etc.

I'm surprised they would diagnose this as overtightening the skewer as I wouldn't think carbon composite dropouts are very compressible. If anything a loose skewer would allow movement leading to wear. I have a Dimond with carbon dropouts and the flame is flexy enough that with some wheels/hubs (Zipp) I can make the wheel shift under load. My cure was to use a DT Swiss twist skewer and rachet it down tight (overtight?!?) which has worked. I've also noted that some wheels/hubs have much more substantial axle flanges and the larger serrated ends have better clamping than small hub ends like on my Zipps.

Calfee has been the go to carbon repair for years. Craig is a super carbon geek and cares about quality. Not just quality, HIGH quality. I met him about 20 years ago & got to ride his "Dragon Fly" prototype for a test drive in Kona. He fixed my buddy's Cervelo P2 and came out fantastic (probably stronger post-fix than original). It was painted and took a keen eye to notice the overwrap near the top/down tube where the snap happened.