There are other early wargames such as the game of war (1858) which uses counters on something like graph paper to arbitrate movement and the Invasion of Britain Wargame 1888 with its draughts like movement. The invasion of Britiain game was the first I have found with its clear political messages of do not build a channel tunnel and Britain is vulnerable to invasion.
Polemos may not keep the title of the earliest model soldier game. 19th Century Europe also had model soldiers and wargames. So perhaps the ‘New War Game of the War in Italy’ from 1860 with its 30mm semi-flats made by Allgeyer, might yet prove to the earliest wargame. I have not yet found a copy, so I reserve judgment at the moment.