I am a native speaker and 'they' is wrong: 'They' should only be used for PEOPLE not things (not even living things that are not human). It should be 'Those are not my pants' or 'These are not my pants'.
I am also a native English speaker. Using “They” for inanimate objects is valid and natural English. “They” is used as the plural of “It” just as correctly as it’s used as the plural of “He” and “She”.