In dbchecker.py the flag to check for expired tombstones is by default set to false in __init__ (check_expired_tombstones=False (Line 59) and later self.check_expired_tombstones = check_expired_tombstones (Line 106)). 
In dbcheck.py the Option is also set to false by default (Option("--selftest-check-expired-tombstones", dest = "selftest_check_expired_tombstones", default=False ... (Line 82)).

Back to dbchecker.py the code in lines 1814 to 1819 is the following:
    def is_expired_tombstone(self, dn, repl_val):
        if self.check_expired_tombstones:
            return False

If I read this correctly, the tombstone check is not performed when the boolean is set to true but is performed when the boolean is set to false. So the check in line 1815 should be the exact opposite (if not self.check_expired_tombstones: return false).