Magento – How does Magento 2 work with CSS grunt-autoprefixer

From looking at a fresh install of Magento 2 (v2.1.8) I see from their Gruntfile.js and package.json files that Magento is using grunt-autoprefixer

1. This is great, CSS autoprefixers are really useful. But I can't see how to use it with Magento's Gruntfile, does anyone have an idea how it works?
2. Also, how would this work in production mode on a live environment? Doesn't Magento2 use use a PHP LESS compiler in production mode, while Grunt is just used for developing with.

Gruntfile.js

/**
 * Production preparation task.
 */
prod: function (component) {
    var tasks = [
        'less',
        'autoprefixer',
        'cssmin',
        'usebanner'
    ]

package.json

"devDependencies": {
    "glob": "^5.0.14",
    "grunt": "^0.4.5",
    "grunt-autoprefixer": "^2.0.0",

I ran grunt autoprefixer and it does not seem to work either.

$ grunt autoprefixer
Running "autoprefixer:setup" (autoprefixer) task
Autoprefixer's process() method is deprecated and will removed in next major release. Use postcss([autoprefixer]).process() instead
File setup/pub/styles/setup.css created.

Running "autoprefixer:updater" (autoprefixer) task
Warning: No source files were found. Use --force to continue.

Aborted due to warnings.


Execution Time (2017-10-29 11:12:01 UTC-0)
loading tasks               145ms  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 30%
loading grunt-autoprefixer  118ms  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 24%
autoprefixer:setup          216ms  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 45%
autoprefixer:updater          5ms  ▇▇ 1%
Total 485ms