I am used to strength being measured in units of stress, such as pounds per square inch, or Pascals.
I am assuming what the author is trying to say is, "this filament will break when holding 18 kg," is that correct?
Material strength is generally expressed in the form of stress but for a component using a load is perfectly logical. Equally while using Newtons would be more precise quoting an ultimate load as a mass is often more practically useful.
What is less clear is whether this is a 'safe working load' (ie you can hang a mass of 18kg off it and it won't break) or if it is the ultimate load.
Safe working loads generally include a factor of safety to account for dynamic loading and quoting the load in kg rather than N sort of implies this but assuming 18kgf as the breaking load would be more conservative in the absence of any other information.