Garden co-op

I noticed that myself.

My best guess is that *technically* you aren't playing "solo" because there's another human being there with you and the AI for Co-Op seems dependent on you actually being alone.

Split Screen, playing with two players on one system, apparently changes the way that the game communicates with the servers. When Split Screen loads you lose the ability to visit the Backyards of any friends and you can't access normal Multiplayer or normal Co-Op; basically you can't interact with any other players except for the one next to you sharing your system. You also can't participate in or complete any single-player quests, which is also true if a friend drops into your Backyard Battleground.

As I see it, as far as the game is concerned you aren't "Solo"in your Garden/Graveyard Op; you are Co-Oping with another person and the AI teammates were evidently intended just to augment people playing alone with no possibility Player assistance. So technically you're considered to be in a Co-Op game where other players are helping so the AI isn't a possibility but you're in Split Screen and can't interact with other players as normal so other players can't join your game so due to what I imagine to be some technical limitation you're limited to a two-player team.