Angel Studios?....I don't recall them having anything to do with Nintendo. The last game they made for GCN was Smuggler's Run Warzones, and now they are owned by Rockstar and just finished up Midnight Club 2. Before that they were making the Midtown Madness PC games for Microsoft.
I believe you are talking about Left Field...
As for this baseball game, I can see one of four Nintendo development houses making it - EAD, Retro Studios, NST, or the new Tokyo Studio.
EAD did make some sports games back in the day, and since baseball is so popular in Japan they could easily make one. They have more than enough people.
Retro Studios had plans for a wide assortment of sports games back in the day before Nintendo cancelled them all...It may be a secondary project.
NST looks like it's having enough trouble as it is with 1080, but they do have enough people to make two games, plus they are located at Nintendo of America which is right near the Mariners...
The new Tokyo studio could very well be making it to...
And of course this would be exclusive since it's gonna be published by Nintendo and the guy who owns more of Nintendo then anyone else also owns the team and the old bigwig of Nintendo of America happens to run the team with him