Can the AI Win a Diplomatic Victory?

I'm not 100% sure, but that is my guess. Every other victory condition is possible. Although a Culture victory can be easily prevented by the player.

That is a great Q!  Please post it also in the bug report area if no answer shows up here.

Of course, just like the player can prevent a Culture victory by declaring war on the about-to-win AI, I imagine the player should be able to break the alliance and prevent any Diplo victory. 

From what I recall from much older threads, I believe the AI cannot win a diplomatic victory. In my games, they usually don't form any alliance (except sometimes with me), but that's because they always seem to find themselves at war with one another...

Yeah, it really was a pretty extraordinary game. I don't think one war broke out between the 9 civilizations the entire time. Everybody was trading heavily with each other and we kept getting anti-war themed planetary council propositions. The maximum war tax was implemented which may have discouraged acts of warfare, I've no idea. There were a couple of grabs at ascension victories with the Torians coming about 50 turns away from winning through that path.

By the time I won a diplomatic victory, literally everyone was allied with at least 5 or 6 of the other main civs.

I believe these were the settings:

Difficulty: Beginner

Map Size: Medium

Civs: 9

Minor Civs: Random but I don't think any were placed on the map.

Rest of the settings were mainly put on random.

It also probably didn't help that we got hit with the Dread Lords event early and then the Jagged Knife planet takeover later.

It might be because of those conditions and the game level?

In my experience, no - there's been a couple of occasions when I've gone for a diplomatic victory where one of the AIs manages to ally with everyone before I do, and the game just kept going. Perhaps the intent is not to "punish" the player for forming alliances.

I'm going to postulate that the "beginning" setting played a big role in this. On the lower difficulty levels the AI isn't nearly as agressive, A completely peaceful game with all 9 major civilizations is otherwise neigh impossible. Good and Evil hate eachother too much, and on the higher difficulties, the AI are also quite motivated to try and take a piece of the smallest fish in the pond.

I'm quite sure the non-AI Diplomatic Victory is by design, although I wouldn't have minded if this restriction hadn't been implemented. I think the first one to ally every civilization should be considered alliance leader, and therefore the winner.

However, there is some logic to disabling it for AI players: the AI will never break an alliance to prevent the player from winning a diplomatic victory (otherwise, a diplomatic victory would be very much impossible). The other way around, however, the player doesn't necessarely feel so strongly honour-bound to remain in an alliance if it means losing the game. Because the player can easily "break" the alliance victory of the AI, you might aswell not implement it. (Actually, it would've been interesting if breaking an alliance immediatly led to war, rather than neutrality, with the remaining allies siding on the side of the "betrayed".) I regard a diplomatic victory not so much a player victory, as more of a co-op victory.