According to the article this was probably an arc flash and they WERE wearing flame resistant (arc-rated) clothing. Most likely these folks will live will little long terms effects. When clothing doesn’t ignite workers are typically not hurt badly. The NESC and OSHA 1910.269 apply here so they should be wearing arc-rated clothing and the best practice is to wear enough to prevent most second degree burns. Sounds like a good quality electrical contractor in that they were wearing arc flash clothing and all the right PPE according to the report.