"Fluorescence microscopy indicated the release of approximately 25,000 micron-sized microplastic particles into one cup of hot water in 15 minutes" is directly from the article. They claim it's not just the ions.
They listed a bunch of things, some of which are in fact bad (although "it's the dose that makes the poison"). But "ions" is pointlessly generic and makes the whole story suspicious (after all what are you drinking when you drink salt water? OMG ions!)