You'd think so, except that the rest of the paragraph doesn't seem to make much sense if this is just a typo, does it? [...] Alternatively, perhaps someone felt that ranking "X and Y" interpreted as "X and therefore Y" as more probable than "X" could be interpreted as "the conditional probability of Y given X is higher than the probability of X"? But that seems like an extreme stretch of the words.

I think that this was the intended meaning. I was also confused by that paragraph at first, but I settled on the same interpretation as you give here. Granted, it means that the comparison should have been between Y and X&Y, not between X and X&Y.

(BTW, I've always wondered whether, given that the conjunction is on the list as an "alternative" choice, subjects interpret "X" by itself as "X but not Y". I've always thought someone would have done experiments to test that idea, but I haven't looked into the literature deeply enough to know.)

The link to Moro 2009 in AlexSchell's comment discusses possibilities like this.