Not sure the relevance of an age in the search for a fact.
While I cannot comment on the ability of glycerine to be skimmed off tallow, as I am unfamiliar with the soapmaking process, the nitric acid part is slightly correct, but not safely correct. In order to make nitroglycerine the glycerine must be added to a cold mixture of nitric and sulfuric acid, the reverse would likely cause an explosion from overheating. Sawdust is not what is used for dynamite, kieselguhr is what is used.
So while plausible it does not tell the whole story
Edited to appease the mods