Humankind is stylised in a very different way to civ 6 bar perhaps the custom avatars which look somewhat similar to Civ 6's leaders from what I've seen of them.
Take a look at Civ 6 units they appear as small bands and pull off lots of over the top finisher kills like the spearman skewering a guy and chucking him over his shoulder, the rifleman leaps over the enemy and shoots him point blank and the swordsman does some Legend of Zelda style sword slashes.
Compare that with Humankind's combat which so far seems to be trying to look a little more grounded with actual formations of men fighting.
I'm quite suprised you don't consider the art direction cartoonish its really seems quite strong to me, putting the leaders aside the units lean quite a bit into exaggeration and a light hearted comic feel like this concept art for the Eagle Warrior:
Given he looks a bit more normal in game but they still kept the exagerated top-heavy muscular build and I think a lot of the military units go for that top heavy build you can go ahead and look at the other artworks for the unique units and I'd say they go pretty heavy into the toony look (the rough riders horse is visibly snarling) For me at least the concept art shows Firaxis was actively going for a toony look. Then of course later we got the Govenors from Rise and Fall.
Anyway I'm aware I'm going off topic here and looking back I really shouldent be suprised at the direction Civ 6 has gone considering the tone was set pretty early and I concede the series always has had whimsy seeping in from the edges (I started with Civ 4 cant speak for anything earlier) it just feels its becoming more prominent as of late.