It's a real phenomena. Mental imagery corresponds to voluntary activation of the visual cortex[1]. The quality of the self-reported imagery corresponds to the degree of activity in the visual cortex[2] while imagining some visual scene. People with aphantasia have little to no visual cortex activity.
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4595480/
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8186241/