I assumed that the 12 years would begin after Trout's current contract expired in 2020, but it will apparently begin immediately.

Trout was already going to be paid $66 million over the next 2 years, so this is really a 10 year extension for an additional $364 million. Given that, I think it is a clear win for the Angels. Of course, Trout now has generational wealth and he gets to stay where he is comfortable, so he probably won't feel too bad if he left tens of millions on the table.