Yes, values require a valuer, and if no life existed, there would be no mental act of valuing anything. Still, what exists could then merit a certain value because of its nature. You're confusing here the mental act of representing a value, with the justification of the value's content. Content can be fictional or nonfictional, for instance, or it can be childishly naïve or nobly realistic. The nihilist's point here is about the semantic adequacy of our worldviews. Just because we exist to value something doesn't mean the value judgment is justified. Some assessments of merit are foolish, while others might be wise. So is the uplifting assessment of our merit as a species justified or not, given the plentiful information about the cosmic context in which none of our accomplishments matters?