Getting a "SyntaxError: Unexpected reserved word" when running wdio with sauce-connect

I've setup a bare test case with WebdriverIO and SauceConnect in this repo https://github.com/gkostov/wdio-test but when running it it fails with

{
          "message": "Uncaught SyntaxError: Unexpected reserved word",
          "filename": "http://192.168.1.51:61964/@wdio/browser-runner/setup",
          "error": "SyntaxError: Unexpected reserved word"
 }

Here are the execution logs https://app.eu-central-1.saucelabs.com/tests/f48c163a8fca4f80b6eb92c17065039c

These are package.json

{
  "name": "test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@wdio/browser-runner": "^8.11.2",
    "@wdio/cli": "^8.11.2",
    "@wdio/sauce-service": "^8.11.2",
    "webdriverio": "^8.11.2"
  }
}

and wdio.conf.js

module.exports.config = {
    // ...
    runner: 'browser',
    specs: ['./test/index.js'],
    user: process.env.SAUCE_USERNAME,
    key: process.env.SAUCE_ACCESS_KEY,
    region: 'eu',
    services: [
        ['sauce', {
            sauceConnect: true,
            sauceConnectOpts: {
                // ...
            }
        }]
    ],
    capabilities: [{
        browserName: 'chrome',
        platformName: 'Windows 10',
        browserVersion: '75'
    }],
    logLevel: 'trace',
};

I'm running it with

# > SAUCE_USERNAME=my_name SAUCE_ACCESS_KEY=my_key npx wdio

There is no code in the test file - I've been removing lines from there (assuming I've been doing something wrong like unsupported syntax, etc.) until I realised that my test code didn't get to even execute. Which is why I set up that test repo to try a minimal case that is still showing the error.

At some point I thought it may be because I'm using the latest webdriverio so it may need to run a in "module" type of project. So I added "type": "module" to package.json, converted the wdio.conf.js to ES6. But it failed with the exact same error.

I've tried looking up examples but only found ones using versions before 8 (and there have been significant changes in 8).