This is just the age-old tension between marketing design standards and product design standards. Apple's marketing materials have all used Myriad Pro since the early 2000s (though Apple only recently got around to using a webfont on apple.com), and thus the announcement is produced according to those standards.
San Francisco is a UI type family for use in products. Just as Apple has never preferred Lucida Grande or Helvetica for marketing (only using either on the web in the pre-webfont era), this announcement doesn't get set in San Francisco. If Apple has a webfont version of it at all, the only place it would get used is on icloud.com, which is an actual web-based product UI.
All that said, this announcement could do with a bit more of the typeface on display than the paltry two letters seen here.
Yeah, historic Apple advertisements for the Macintosh were in the tall, thin serif Apple Garamond font [1], but the system font for the Macintosh was Chicago, a less-tall-proportioned, heavier sans serif.
I would have appreciated a type sample or maybe some mostly-vapid sketches of a few letterforms, but I didn't really expect the entirety of the text itself to be set in the system font.
(I miss Chicago. I kind of want to see a hacked up OS X using Chicago or at least Charcoal as the system font....)
San Francisco is a UI type family for use in products. Just as Apple has never preferred Lucida Grande or Helvetica for marketing (only using either on the web in the pre-webfont era), this announcement doesn't get set in San Francisco. If Apple has a webfont version of it at all, the only place it would get used is on icloud.com, which is an actual web-based product UI.
All that said, this announcement could do with a bit more of the typeface on display than the paltry two letters seen here.