He said dust. I still don't think that there was any way I could have been expected to know that he meant 'elements of the earth'. And, I did check to see if dust was used differently in religious circles than it is normally, but the results I found gave me the normal meaning that I interpreted it as.
EDIT: I've researched further, and find bizarre, to me, interpretations of dust but not the one that you mention.
Spiritual Meaning of Dust NETBible: Dust (the latter says that dust is the material out of which God formed man, but doesn't specify that that isn't the normal meaning of the word 'dust'.
If he had said that we are 'made from the elements of the earth', then that would be clear.
Given what I've found when I've researched this in hindsight, I still don't think that even in a Christian context, that I could be expected to know what he meant.