They do have open source projects, but their mission is not to necessarily be an open source company. They want to equalize the footing in AI for everyone, and this might require some degree of secrecy so that commercial AI companies do not get an edge. At least, that is my perception, feel free to correct me.
I think its more about making the same research available to everyone, or other things that are relatively free to distribute (like code, even though, as hinted, it does not mean that everything will be open-sourced as its written).
Although, you do make a good point. If only the most powerful companies can afford meaningful machine time, how democratized can AI be?