I think meaning and existence are a matter of semantics. The quadratic formula is a fundamental tool of mathematics? Does it exist? Not materially, in that it's one statement in an abstract language. Similarly, while we can demonstrate that gravity is a consistent force that acts upon us, and even have a law (some mathematical rules) that accurately model how objects behave under the influence of gravity (accurate enough to fling space probes to distant planets), the theory of gravity is only a description of the material (very similar to Ceci n'est pas une pipe ).

When a nihilist asserts that knowledge, morality and meaning don't exist, that is to say they don't exist much the way the quadratic formula or the law of gravity doesn't actually exist. While we can understand gravity to a high degree of accuracy, we cannot know the future outcome of a gravitational event (say, the flight trajectory of a space mission) until we see the end results (and even then we're only assessing what happened based on how we interpret the evidence of the aftermath.)

None of this is to say we cannot invent our own meaning (and for social purposes meaning and mores can be very useful!) but they remain in the abstract. They might inform what your intentions are, and what actions you choose to take, but until after you've taken action, all of that is imaginary.

Because we are highly social creatures and organizing is one of our superpowers, we take interpretation of the real, and the meaning we derive from it very seriously. We also take mores very seriously (to which I point to the United Nations Universal Declaration of Human Rights, which highlights how we should treat refugees (id est, the least of us ), but as grandiose as such a charter might be, it only exists as an idea that we communicate to others and decide whether or not we agree to treat each other in kind. But the ephemeral nature of these social constructs becomes very clear in the jungle, when microbes, pathogens and hungry predators all ignore our rights, mores and higher purposes and descend upon us without hesitation.

Much the way King Cnut's royal authority was not recognized by the sea foam, meaning exists only so far as those who observe and regard it.