The sentence is being sarcastic. Think of what a blow torch is, and how loud and amazing it is to have a flame shoot from the nozzle. Then think of the word "subtle". If you compare the two, there is nothing subtle about a blowtorch. Therefore, the guys "irony" was actually very obvious and not subtle at all.

It is an ironical comparison. A blowtorch is not subtle. Therefore this sentence means: "His rhetorical skills (in this case: irony) are very clumsy."