That is some very weird reasoning, though.

"Looking at their statements again, if Guard 1 is telling the truth about guarding diamonds (as we deduced), he would be lying about the silver. This is okay, because Guard 1 can tell a half-truth while guarding diamonds. For Guard 2, if he's telling the truth about the silver, he'd be lying about the platinum, which is also allowed. So the treasure they are guarding can be diamonds. This makes Guard 1's statement (The treasure is either silver or diamonds) half-true and Guard 2's statement (The treasure is either silver or platinum) half-false."