GM announcing this means they have secured their battery supply. Likely they feel there's a scramble for supply coming and they didn't want to be left behind. They also must have looked at the future of the technology and like what they see.

To secure supply they would have had to make a major deal with China, where most of the world's lithium gets processed. GM also sells more cars in China than the US (though less revenue). That also likely factored in the decision since China has announced aggressive EV targets. Given GM's integrated global production they would lose advantages of scale by having a split lineup of EVs and ICEs.