Correct, and I'm not an expert on RF theory but did a lot of research to select an appropriate feedline. There is a good writeup here https://www.bridgecomsystems.com/blogs/bridgecom-official-blog/how-to-select-feedline-for-a-repeate which is primarily what I based my selection on.
The article recommends against any LMR-### or LMR-#### for duplex service, and to instead use Superflex, Heliax, RG-214, or RG-393 on the antenna side of the duplexer. I don't know if a copper clad aluminum core will be an issue but something like M&P UtraFlex 10 is a good alternative to heliax and not much more expensive than a quality cable of the types listed above.
I've experienced duplex grunge testing an RT97S with a Nagoya UT-72 antenna which is about 15 feet of PL-259 edit: RG-58A/U. I believe that coax is a copper clad aluminum center and copper clad aluminum edit: tinned copper braiding. After releasing PTT there would be a low pitch screech which would sometimes continue activating the repeater for a few seconds.