We have a similar system where I work except the dust being removed is from an electric arc furnace used for melting steel
All the media reports have said they were opening up an access hatch to look for the fire when it blew out at them. Sounds like an air (oxygen) driven event to me rather than a heat driven flashover.
Would the rapid fire growth also have something to do with sawdust being so easily ignitable?
Not questioning any tactics used, just looking for lessons learned. I hope the reports of him getting out of the hospital in a few days are true. Get well soon Brother!