The repeated access to string is cache friendly, so has a good chance to be stupid fast.
On the other hand, memory allocations are stripped to (what seems to be) a minimum.
The clever avoidance of count and no intermediate collection to store the counting is appealing to the audiance. :-)
Also, codewars community tend to favor 'less code' approach. :-))
That's my understanding, but would like to hear someone more experienced in Rust.