Funnily enough, I have very vivid dreams so it really baffles me how my mind can just shut that part of my brain off when I'm awake haha ^^
I think that it CAN be caused by trauma but isn't ALWAYS caused by trauma; for me, I was born with it and it was genetically passed down to me and my sister by my mother (I have no clue about my father). We have varying degrees of aphantasia as my mum has on a scale of 0 (seeing nothing) to 10 (seeing vividly) she is probably at a 2 whereas I'm fully at 0 and I believe my sister is at a 0 or 1 also (we don't discuss it frequently). Anyway I'm getting off topic - I believe that trauma can sometimes cause aphantasia or physical trauma like a head injury can but it's not always the case as it can also be genetic and happen at birth or develop through childhood. I don't ever remember "seeing" things with my imagination or really having any other senses in my imagination besides sounds of things I've already heard but not always able to control. I thought "seeing" images in your head or a maths equation was a metaphor used in media to portray ideas of what the character was thinking visually without realising that it was also somewhat literal too - I never understood the allure of reading or getting lost in a book or fully understood daydreams as when I would "daydream" I'd only be lost in thought and not realise I'm looking at someone as I'm too deep in thought. It was an incredible awakening to realise that others could actually truly imagine anything and that made me slightly jealous. AH I keep getting off topic! Overall, I believe that what you're saying has some truth to it but it's not the case for everyone :)