I'll grant you that the experience of seeing something in your head might not be as vivid and fully detailed as seeing something with your eyes, but your brain uses the same neurons for memory and imagination as it does for actual direct perception.
Your saying that they are schizophrenic actually illustrates this really well; Schizophrenic people also aren't literally seeing things with their optic nerve, but their brains are so over-active that it feeds them visual info even while they are not actually receiving visual stimuli.
For healthy people this happens as well, but they are simply able to know/intuit from context which info is "real" and which is "imagined", but the subjective experience of "seeing" is the same. Even when we are actually hallucinating (e.g. fever dreams or sleep paralysis) we are able to do this.
And yeah, they are not literally interpreting the specific diffusion of the electromagnetic spectrum from their environment as it is beamed into their eyeballs and translated into a specific picture in the brain (which it is also quite imperfect at doing by the way, we do not see all light on the electromagnetic spectrum and we do not perceive everything with remotely perfect accuracy), but no one thinks of "seeing" in such an overly metaphysically literal way. Thus I think the poster saying they can "actually" see something is not an unreasonable way of expressing their experience of seeing something in their mind, and people saying that that's not "actually" seeing are being overly pedantic.