I've seen that one of the best ways to loosen a seized nut is to heat it with a blowtorch. I have a cooking blowtorch which claims to reach 1300C but also says "for cooking use only". Is there any reason it would not be suitable for DIY purposes?
This is merely restrictive language from the legal department, so you don't win a lawsuit after blowing up your garage while trying to loosen a nut on the lawnmower with the gasoline leaking everywhere.
A torch is a torch. Use common sense so as not to get into a situation involving a lawsuit you'll lose due to violating manufacturer's instructions.