This. If you ask me to imagine a triangle I'll start thinking about having three sides and three angles and the area is half of the base times the height and it's a rigid body and the angles add up to 180 degrees... but there's no visual aspect to it.
Sometimes I wonder if aphantasia gave me an advantage in mathematics, because I had no trouble whatsoever with the concept of "abstract symbols".
I lack imagery but also am nonverbal and hit limits in math. I am terrible at rote memorization or application of formulae.
I did well in math up to a point, which may have hurt me. I simply felt answers to quite a bit of algebra and some calculus, but it doesn't scale. I also got accused of cheating a couple times in primary school, when I could not explain my work when arriving at an answer very quickly.
I should clarify that I can still imagine what a room looks like and what’s in it. I just don’t see it. It’s more like I feel the layout or know where things are, almost like navigating a mental map without any visuals. Specific details like colors, patterns, etc. are much harder to recall unless I am intimately familiar with the object or whatever.