Not even that. For example if there are 10 assembly lines in parallel that the slowest step takes 60 seconds, you still get a phone each 6 seconds

Xiaomi said assembling a smartphone from circuit board to finished product takes about six seconds, and, at full capacity, the factory could produce more than 10 million devices annually.

That sounds like the final assembly takes six seconds, which is entirely plausible given how fast robots move.

That was my reaction to the headline. Then I read the article:

Xiaomi said assembling a smartphone from circuit board to finished product takes about six seconds, and, at full capacity, the factory could produce more than 10 million devices annually

They are indeed stating each phone only takes 6 seconds to complete

With ~30 million seconds in a year, their output is 1 phone every 3 seconds

Also that’s not that impressive. 30 million devices is roughly one per second year round. Phone manufacturers make many times that in a year.