The article itself provides an explanation without needing to invoke the phantom of pervasive fitness declines:
Harmful mutations like these are predicted to build up in small, isolated populations that become inbred, a phenomenon called genomic meltdown, according to most evolutionary biologists. If your options for a mate are limited, you can’t be too choosy about undesirable genetic traits, so those don’t get weeded out. The Wrangel Island mammoths provide a rare opportunity to see that theory play out in a real population, Rogers says.
To provide evidence of a more pervasive pattern of declines, Paul would need to provide an example that is not from “small, isolated populations” that may be subject to inbreeding.