Explanation: 'They/Them Army' is a somewhat tongue-in-cheek reference to the increasing acceptance of LGBTQ+ folk in national militaries in the West in the modern day. Rather than being derogatory, it's generally meant in mockery of the hyper-macho obsession of some ultranationalists, who seem unaware that one's pronouns have very little influence on one's ability to deploy military force against a target.
The image displayed is that of WW1 British tanks - which were designated 'male' and 'female' according to their loadout of weaponry - 'male' tanks having cannons, while 'female' tanks were loaded down with machine guns. As it was a time of great experimentation in military equipment, going half-and-half was eventually decided as the ideal configuration - making the 'hermaphrodite' (it was a... different time) tank the standard by the end of WW1. They/Them Army wins again!