About 11 minutes in as an aside they look at an example where they're initially mutating a growable array (Rust's Vec, Mojo's List) and now they want to replace it instead.
The narrator (rather than Chris) shows that Mojo makes this fairly easy - we were never promising only to mutate the container, so if we replace it instead that's fine. And they (presumably correctly, I am not a Python expert) say that Python just can't do this.
But they also claim Rust can't do it either, and propose a fairly serious surgery to adjust the function signature instead to write a function which takes and returns ownership.
But you don't need to do that in Rust. We have a mutable reference, Rust doesn't mind us changing what it refers to, it's mutable after all, we just can't move it because it's not ours to move. This is where core::mem::swap, core::mem::take and core::mem::replace enter the picture, they allow us to easily use this capability.
For the example they're explaining you can just core::mem::replace(v, vec![8, 8]); and you're done, the signature doesn't change at all.