"The Epstein-Barr virus (EBV), which resides silently inside the bodies of 19 out of 20 Americans, is directly responsible for commandeering what starts out as a minuscule number of immune cells to go rogue and persuade far more of their fellow immune cells to launch a widespread assault on the body’s tissues, the scientists have shown."
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EBV resides in 19 out of 20 Americans (95%) ... but it is estimated that only 5-10% of the population in the United States have autoimmune disorders. The prevalence of Polymyalgia Rheumatica (PMR) in the United States is only about 0.5% to 0.7%, based on recent studies.

There must be other factors why the vast majority of people with EBV don't develop autoimmune disorders. EBV might be one trigger but other infections can also trigger autoimmune disorders.

I believe that infections can "trigger" abnormal immune responses. I have an autoimmune disorder that derives its name from a reaction to an infection. It is called Reactive Arthritis.
https://www.mayoclinic.org/diseases-conditions/reactive-arthritis/symptoms-causes/syc-20354838
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There is a genetic link to Reactive Arthritis. It is called the HLA-B27 syndrome.
https://www.ncbi.nlm.nih.gov/books/NBK551523/
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The demographics of autoimmune disorders are fascinating. Having one autoimmune disorder is a risk factor for others. There are at least 100 known autoimmune disorders. EBV might account for some cases of autoimmune disorders.
https://newsnetwork.mayoclinic.org/discussion/new-study-calculates-autoimmune-disease-prevalence-in-u-s/