It's an adjustable rate mortgage, not a fixed one, and that is the starter teaser rate. You don't know how long it is in effect for, what the mark-up from underlying benchmark will be, and what sort of cap on rate increase there is.
IIRC, Carolyn Warren's Mortgage Ripoffs book said the low teaser rate, pick a payment, option arm that got sold to "subprime" borrowers was originally conceived well before that for really wealthy clients so they could gain liquidity to deploy money elsewhere and earn higher return.
In Zuckerberg's case, my totally unresearched guess would be that this mortgage is somehow designed for tax purposes (tax deductions).