Adding static typing to a dynamic language means that more work must be done at compile time than at run time.

One appeal of dynamic languages is you don't have a long compile time[1].

So I imagine they emphasize this to address the worry that this will get ruined and there will be a horrible workflow.

[1] However Turbo Pascal was a one-pass typed compiler in the early 80s that was blazingly fast with 1 MHz CPUs and 64 KB RAM. :)