A man can't get pregnant, you can't go out into the outer space without a space suit, a person can't reach the speed of light, because our bodies will not survive this and we could go on and on, but let's be more down to Earth. If nothing is impossible, why don't you break some records?
Yes, you have made a reasonable point. There are some eternally impossible tasks in the world, which cannot be done by any creature created on earth.
The phrase 'Nothing is impossible' does not apply to this eternally impossible task. This is a motivational sentence. Work that is possible or common for one. But there are people for whom it is almost impossible to do the same simply because of his disability. To do the impossible with full confidence based on strong aspirations is to make the impossible possible. The sentence is associated with this example.