Yeah that’s what I thought too. The horrors are described well, they just typically don’t get described through their physical form. As you say, because the human mind cannot comprehend it. There is a lot more focus on impressions, comparisons, and effects, rather than on a real physical description. Personally I thought it was quite neat!
He also called them mûmakil in elvish. In my mind, when the Hobbits call them oliphaunts it is because a long time ago someone talked about elephants, and over the years the correct pronunciation was lost.