The problem here is that your mixing up techologies.
Out of the box, the kirby webpack uses a built in PHP server. It does not use Valet, but it does use browsersync.
There is a proxy setting in the config that if you set a local domain name like
'yoursitename.dev' it will switch off the built in PHP server, and expect you to have your own local domain running on a local server, which might be via Vagrant or mamp.
I have not tested it with Valet.
The snippet you posted is for Larval Mix which is something else entirely, unrelated to kirby webpack.
If you do have local server running, open up
main.config.js and change line 36 to match your local domain name. Re-run Kirby webpack and it should proxy that domain.