I really don't recommend this practice, since it can be extremely dangerous and is not compliant with the NFPA 1403 Standard on Live Fire Training Evolutions.
Years ago, I worked in a jurisdiction that made a routine practice of using diesel fuel to help simulate "roll-over effect" during live-burn training. After a full day of instructing the burns, and in the middle of an evolution, I remember looking down and seeing my entire diesel-soaked arm on fire!
Needless to say, it wasn't a good feeling. I was very fortunate that my properly worn, and NFPA-compliant, structural firefighter protective clothing and SCBA gave me enough time to quickly, and I mean quickly, extinguish the fire without becoming injured. The gear, however, was destroyed and had to be replaced.
As this story demonstrates, and my own experience reinforces, flammable liquids and turnout gear do not mix.
In my example, the presence of a combustible liquid was well-known, and obvious. In Firefighter Horst's case, there was no reason to suspect the presence of flammable liquids in what appeared to be an otherwise routine building fire. Thankfully, both firefighters reacted fast, kept their composure, and were unharmed. It will be interesting to hear what the spilled material was, when the investigation concludes.
So what's the takeaway?
Nothing new, but everything important: expect the unexpected; maintain your situational awareness; wear all your gear properly and completely; and keep in close communication with your crew in case they, or you, need help.