Apple joined in Oct 2012. It could be that other companies have agreed to join and were allocated those codes, but are not yet up and running as of the date the slides were made. Just speculation. I recall that there was a slide saying "dropbox coming soon", so they are probably working on onboarding other companies.
With only two characters for the source ID, somebody at the NSA is thinking long term and using more than just digits from now on. Many more providers will come in the future.
And though I'm dying to know just how deep the rabbit hole is, I'm thankful that the slides are being steadily released. In today's world I guess that's the only way to grab attention and gain momentum.
It also helps eliminate the credibility of the government and its corporate collaborators when they disclaim participation based on subset x. When subset y, released the next week, points out that they were in fact aware, then the people who are exposed make stupid statements such as the lovely,"We don't spy on US citizens."
I'm wondering what this theatrical sleight of hand is keeping us from seeing elsewhere. We've probably invaded Iran or sent another trillion dollars to the banking industry.