It's someone fundamentally misunderstanding an actual fact (that blue colors in nature almost never work the same way as normal pigments, and are a physical structure refracting light instead of a chemical that absorbs other light frequencies and reflects blue ones) as meaning something isn't "really" blue. Like the feathers aren't a blue pigment that could be dissolved in some base and used as paint (presumably) because their color is a structural rather than a chemical property, but it's just silly to decide that the ontology of something "being a color" is dependent on chemical pigments instead of what it literally looks like when exposed to light.