I was struck by this also. Why would you release a brand new font that you plan to spread widely and use in all your products, and set the announcement in a different typeface?
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....)
Apple’s marketing materials use Myriad. It would make no sense to set their marketing materials in San Francisco. That would be totally inconsistent and weird. (Also, this is just a download page for people wanting to work with the font … not a place to show it off. I mean, if this were a place to show it off then setting that in San Francisco would still be wrong, wrong, double triple wrong. You would set the text in Myriad and give examples of San Francisco.)
Makes no sense.