I don't think you can compare Zuckerburg's refi with the 3/1, 5/1, or 7/1, etc. ARMs that are readily available for all of us regular folks.
You may be focusing on the original low teaser rate before the ARM becomes fully adjustable later down the road. Zuckerberg's ARM is freely floating right off the bat (sounds like 1 month LIBOR + 0.8%).
Zuckerberg can probably directly go to a wholesale lender, who may be looking for 1% profit over their underlying cost of capital.
Plus, we don't know if Zuckerberg paid points to buy down his rate, and we don't know what appraisal of underlying collateral (home) is and what his equity in home is based upon requested refi amount.
Doesn't sound like he is getting some sweetheart deal that lender is losing money on. They are probably making more money than short term Treasuries or commercial paper money market type funds that might be lending to European institutions, and presumably risk of Zuckerberg defaulting is next to nil.