They reach this conclusion using a version of Dembski’s improbability argument. No, this is not Poe and it is not self-parody. It is for real. (You can visit the Uncommon Descent page that makes this argument, here, and I have used donotlink so that they will not benefit from your visit.). The improbability of any given dream is suitably enormous (it is) and specific (is what it is, not anything else). It follows, by the application of Dembskian statistics, that every dream is Intelligently Designed.
The author, apparently one N.Kendall by way of Barry Arrington, boasts “This is just one of several disproofs of materialism that I have tried out on atheist websites. Never once had anyone lay of glove on it [sic]”.
So there you are. Prepare to be converted from your sinful materialist ways. Laying of gloves optional.