You can try to explain it away any way you like, but the fact remains that Civ VI's AI is incapable of using its dozens of units to take a single target, which is the ONLY military target on the entire map. There is no excuse for this. And OP is playing on a Duel map, which is not bigger than Advance Wars ones. The unit variety is also actually much greater in Advance Wars - most of Civ VI's units are simply upgrades of other units(which are treated identically by the AI), while all units in AW are unique. The AI is predictable in those games, but that's because it takes smart options instead of moving what seems to be completely randomly.
Feel free to not respond, I'll take that as an admission.
Moderator Action: Please do not troll. leif
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There's still no comparison. Advance Wars is a pure wargame where peace isn't even in the picture. The AI has but a single opponent, a single objective, and most campaign missions are quite scripted.
You won't see it performing complex feats even Civ AI has trouble with, like old school amphibious invasions (with actual transports) or making effective use of aircraft carriers unless it's been explicitly scripted to do so in the specific context of unequivocally fixed missions and maps. Hell, even combat is simple since it's always unit-on-unit, with no more complication than terrain in some cases. Furthermore, on many missions the AI begins play with a fully formed army composition and a solid foothold, and the challenge relies more on bringing down that wall than withstanding coordinated, unscripted attacks by said force.
Beyond all that, the AI in Advance Wars doesn't have to trouble itself with research, developing cities, expanding, evangelizing, improving the land, diplomacy, etc. etc. in addition to waging war. Civ AI's Achilles' heel tends to be prioritization in the face of having so many things to juggle, which is why it's not uncommon to witness its indecision.
The first series of tests throws the AI into a war it didn't want nor prepare for, and it's expected to conquer a city it may not even want. It is caught off-guard by the declaration, and it unsurprisingly flounders if one takes a moment to consider the context regardless of the subjective goal the human player had in mind for the experiment.
The second series of tests, perhaps more representative of the AI's reality, produces much more promising results. It shows a more decisive AI mainly due to the fact it seemed to want the conflict and had prepared for it.