I'm quite sure that the WWI scenario at least will have some own units. You don't want to have Musketmen or Infantry as it looks right now fighting in that scenario. I doubt that it counts towards the 18 units though - the Huskarl and Janissary weren't advertised as new units and it would be stupid to do so.
A thought occurred to me while looking over the statements about the WWI Scenario. It is supposedly based on a 'Guns of August' scenario in 1914. IF the scenario has scenario-specific units that are accurate (never a sure thing with Firaxis, admittedly), then the German and French infantry would both be in uniforms virtually identical to what they wore in 1870: French in dark blue with red trousers and kepi, Germans/Prussians in
pickelhaube helmet, dark blue (or early
feldgrau, but in Civ VI scale that's scarcely noticeable) uniform and jackboots.
That means, regardless of Firaxis' intentions, both units could also represent the black powder Rifleman as well as the 'Great War' infantry.
Just a thought to start some Modders salivating...
The only thing that makes me slightly optimistic about this is that they have specifically related the scenario to the 'open field' battles of August-September 1914, before the ugly trench warfare set in, and the traditional 'look' of WWI infantry with steel helmets and drab uniforms didn't start until a year later.
For the trivia-pursuit among you, the British 'Brodie' helmet, the one used for GWI in Civ V, was adopted in late 1915 by the UK and in 1917 by the USA, the French 'Adrian' helmet also adopted in late 1915, and the German 'coal scuttle'
Stahlhelm wasn't issued until 1916, so there's no legitimate reason to have ANY of them in their WWI scenario as it has been described.