Why does he use "it's" and "that's" at the same time? Is it commonly replaceable?

Native speaker’s answer
Rebecca
No, "it's" and "that's" are not commonly replaceable or interchangeable. He most likely uses "it's" and "that's" right after one another here to add emphasis on his elbows being too high. If he would have only used "it's" twice, then there wouldn't be as strong of an emphasis. So using "it's" and "that's" together can add emphasis when restating something although it may not be the most correct thing to do.