What they claim is that if all editing stops, then after a period of time everybody will be looking at the SAME document. This is what is meant by "eventual consistency". Achieving this in general is indeed a difficult problem, but (some of) these algorithms do solve it, though it can be tricky to correctly prove that they do. I agree that it is not possible to ensure that the document everybody is looking at is what they actually wanted it to be. However, there are some options for what happens, where some results may be technically correct -- we are all looking at the same thing -- but obviously really bad. This beautiful talk has some examples: https://www.youtube.com/watch?v=x7drE24geUw