Could be plaster and lath also.

There is also a difference between the concept of fire resistance and interior finish rating. One is associated with the full thickness of the wall or ceiling assembly and one is associated with just the finish materials. The NFPA Fire Protection Handbook provides finish ratings of many materials including plaster and lath.

There is a lot of data and videos of compartment fire tests that go to flashover in 3 to 5 minutes without any the use of any accelerant, so this fire chief's opinion is unreliable.

May want to also consider an alternative hypothesis of a fire starting in the ceiling assembly and dropping down, if the timing based on flashover and ceiling failure is disproved.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com