Keyword is imagine, because with Mojo you do have to write your code in a new language. You cannot use the Python parts if you want speed. That's writing code in a new language.
That's the stated goal, it's not mature technology.
You can have your python parts source compatible with the new language.
Of course they are far off from that.
But not that far off if your python code is typical ML framework consumer code.
Try to do it yourself (say take a merge sort implementation and try to make it scale), it'll be roughly this much after a few hours of work.
Now imagine the compiler is doing this for you.