Ah ok I see. IterTools.step is depricated, so one should use std.step.by() instead, and then no imports are needed.

I just retested, removing extern crate itertools is possible, but if I remove use itertools::Itertools; I get the error:

error[E0599]: no method named `step` found for struct `std::slice::Iter` in the current scope
 --> src/lib.rs:4:22
  |
4 |         array.iter().step(2).sum(),
  |                      ^^^^ method not found in `std::slice::Iter<'_, u32>`

error[E0599]: no method named `step` found for struct `Skip` in the current scope
 --> src/lib.rs:5:30
  |
5 |         array.iter().skip(1).step(2).sum()
  |                              ^^^^ method not found in `Skip<std::slice::Iter<'_, u32>>`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0599`.
error: could not compile `challenge`


NOTE: Line numbers in error messages can be incorrect due to concatenation.

I would guess that when the solution was made extern crate itertools; was nessesary. Rust 2018 removed the need for extern crate in a lot of cases.
Thanks for the comment!