You do not actually have to define "truly random". You do have to trust that the mechanism you are using to generate keys (radioactive decay or rolling dice, for example) is unpredictable.

There is a more general point here, that sometimes seems to be lost in philosophical arguments: reality doesn't pay any attention to the meaning of words. If "unbreakable" is not well-defined (I am not sure that is so), then it is a problem within the domain of language, not cryptography.