Fat burns readily when heated, it's called the wick effect

"There are no bodies because they cremated them all to dust."

"Where did the fuel come from for that?"

"Well they don't need that much fuel, they had crematoriums."

"Well that doesn't seem possible and the evidence you're showing is burn pits, not even crematoriums."

"That is because they did most of the burning outside."

"THAT REQUIRES EVEN MORE FUEL SO WHERE DID THE FUEL COME FROM?"

"They don't need much, they had fat jews to fuel specially designed magic crematoriums!"

"What? You just said they used burn pits! Now magic crematoriums? This is implausible on its face! Where's the evidence?"

"Well here is a memo from a guy who knew a guy that heard in a bar that it happened, and a patent application from after the war!"

"What? That isn't evidence, just some statements we can't verify. Show some photographs or maybe-"

"All the evidence was destroyed so you can never prove me wrong, ever, just trust me on this bro!"
Did you claim that the fat on the jews body accounted for the lack of fuel available to properly cremate the bodies?
The only 'advantage' to these guys all quoting from the same handbook is that you can do the same right back at them, and for this case I have a nice effort post regarding the use of human fat as fuel.
You could theoretically take a fat human cadaver, butcher it to extract all the fats from the corpse in a gross manner, take those fatty deposits, render them with fine grinding and then liquify with heat until you have sterilization and good flow. From there you proceed with a transesterification reaction (on which there is very little research regarding human fats for the obvious reasons), but we can use the processing of beef tallow as a handy stand-in. Due to the high levels of free fatty acids in mammal tallow (tallow is purified and rendered fat) you need an initial esterification process with an acid like sulfuric acid for example (very common industrial acid) to decrease these proportions and then drain the resultant waste glycerol. Then you can move to the second stage, where the tallow can be mixed with whatever catalyst you have handy and methanol, (by the way this stage requires constant heating and stirring over a period of time) to allow for efficient transesterification. At the end of this process you have a liquid fuel that can be blended with more traditional diesel fuels to run diesel motors, heaters, etc. Also this process was only developed and industrialized recently, in the 1980s and is only really economic in areas where there is a ready source of animal fats (in livestock rearing areas that produce lots of fat waste) and where you also want to reduce reliance on traditional petroleum based fuels. So likely not the case, barring some magical secret German patent that somebody's brother's cousin's coworker's former roommate wrote about while under Soviet interrogation.
At a more reasonable point (and I use the term very loosely), we might task some of our theoretical Sonderkommandos to butcher out all the fat Jews from the Netherlands to harvest their fat (presuming that we have a steady supply of said Jews at the rates and obesity levels required) and then taken the raw fat and tried to use it as fuel. Pure tallow is not an easy fuel to work with, but presumably you could fill troughs or vessels at the base of the crematorium with fat, liquify it with heat from your coal/natural gas/diesel burners and attempt to ignite it once liquified. This presents a number of engineering issues, chief among them is that, unless rendered down, raw fat is highly variable in composition and behavior. This means that there can be some very unpredictable performance; first a problem because you want
steady, controlled heating and secondly it could easily result in damaging the crematorium's primary burners or ruining the secondary fat burners. Any possible gains in efficiency could be very easily lost in maintenance trouble and of course the capital costs of designing and retrofitting the crematoriums to work with animal fat.
Alternatively you could simply try to combust the human fat without processing at all, which means that it is still distributed throughout the corpse after loading in the crematorium - which presents similar issues of unpredictability, inefficiency and possible damage to the crematorium. With those fat deposits locked in the cadaver, it is difficult (or at least impractical) to determine the total amount of fat and when it may combust - so how does the operator know when to reduce the fuel flow to let the fat burning 'take over' and for how long? Does he just reduce the fuel rate over the entire period, or cut it off during the flare? (It may surprise you to know that it is not efficient to just dump a load of fuel in all at once, and instead better to deliver it at a controlled rate.) Then we have another issue that comes with a rapid, energetic combustion of a fuel source that may still be wrapped inside some pressure containing vessel like say...dehydrated skin. The rapid buildup of pressure as the fat combusts can cause an explosive over-pressurization which could be damaging to your cremation unit - for a traditional single fixed tray unit this may not be a very serious concern, but for complex and mechanically convoluted designs like the ones purported to exist such events could severely damage the unit and take it out of commission for some time.
Perhaps I misspoke earlier, but I'll clarify; the use of human fat as a combustible fuel is theoretically possible, but not practical or feasible in the time (1940s) or manner that has been proposed by these retards