Wikipedia has 777 on order, so I guess it's not a typo. Two others sources give 764 [1] and 780 [2] orders, respectively.

Given that Air France recently converted outstanding orders for the A380 to three A350, that would make 777 as of last month consistent with 780 as of today.

This 780 figure is also including two orders for a total of 16 A350-800, which will never be built. So it is consistent with the 764 figure that do not include these A350-800.

Conclusion: not a typo.

[1] https://sites.google.com/site/a350xwbproduction/orders

[2] https://docs.google.com/spreadsheets/d/1tSkBafqSz-xEdM1VLbQy...

Edit: added the AF recent order, and the A350-800 adjustment.

Maybe it's a SEO hack.

Perhaps they had sold >777 by the end of the month, but they happened to send out their press release after their 777th sale.

777 is also a number. I understood it sa they received 776 + 1 orders, although yes ironic.