I had conversations on this subject with a friend who has no mental monologue, I don't think you're correct. I think we get by through making use of the thinking processes we have available, but that doesn't mean that there's no major differences in how we find out our answers, just that we tend to get to roughly the same answers no matter which route we take.
For one thing that doesn't take into account how long it takes for your thought process to reach conclusion, if you're incapable of creating mental imagery and have to solve a spatial problem through mental monologue alone might that not take longer to intuit?