: : So, there is no real reason to suppose there were no swamps in Mordor, just that the characters of LotR probably never saw them.

However, this thread was brought up because members of the movie said they were in the "swamps of Mordor", and while there may have been swamps in Mordor, as you say, "the characters of LotR never saw them". And since we know that Jackson is, too some degree, trying to remain faithful to the text, that allows for two possibilities:

1- Jackson has added swamps to Mordor which the characters will see

2- Sean Astin was mistaken and instead referring to the Dead Marshes.

Since, as you point out, the characters never see a swamp in Mordor, I would like to assume that Sean Astin is merely mistaken in his statements.

-RR