It's very true for me. I think it's a natural part of having predominantly non-verbal thinking, regardless of whether one is autistic or not. It's difficult translating imagery into words, and even more difficult when there are no images, just silent emotions in an otherwise "empty" mind. I constantly find myself stumbling over my words, searching for the right ones because they don't come easily, and often failing to find them. Because of that, my sentences are usually not too coherent, and I sometimes use gestures to fill up the gaps and make it easier to get my meaning across. (It's especially true when I talk about things which I can visualize very vividly, usually some objects. Say, if I'm talking about a triptych, my hands naturally start showing the shape of it - palms against each other in front of me, then moving away from each other, and sideways at an angle; when I'm saying "holes in the board" describing a board game, my finger will draw circles in the air in front of me, or on the table, and when I say "those plants, with winding stems", my hands will twine showing the shape of the plants. It's somehow easier that way, when my hands trace the outlines of the image I have in my mind).
With writing, when I have time to think about what I'm saying, and can correct it as much as I like, it is a lot easier. I also noticed one thing. I'm often not sure how to describe the mood I am in, that is, to me it's very clear but I hardly know what it's called - it usually happens with emotions that cause a physical reaction (anxiety, panic, fear, worry, etc), which I have difficulty telling from each other almost all the time, but sometimes I can't even say for sure if I am cheerful or sad. But, once I work through it deliberately in writing, and think carefully how to describe my state of mind and the way it makes me see the surrounding world, I realize what the feeling is. That's why I find keeping a journal very useful - it is a great help in defining my own feelings for myself.