1-install.sh

composer require a5hleyrich/wp-queue

2.php

class PostSaveJob extends \WP_Queue\Job
{
    protected $postId;
    public function __construct( $postId )
    {
        $this->postId = $postId;
    }
    public function handle()
    {
        $post = get_post($this->postId );
    }
}

3.php

class PostSaveJob extends \WP_Queue\Job
{
    protected $postId;
    public function __construct( $postId )
    {
        $this->postId = $postId;
    }
    public function handle() {
        $post = get_post( $this->postId );
        if( ! in_array( get_post_type($post), [ 'post', 'page' ]) ){
            return;
        }
        $controller = new \WP_REST_Posts_Controller( 'post' );
        $route = 'wp/v2/posts/' . $this->postId;
        $request = new \WP_REST_Request('GET', $route);
        $request->set_param( 'id', $this->postId );
        $response = $controller->get_item($request );
        $name = sprintf( '%s/posts/%s.json', __DIR__, $post->post_name );
        file_put_contents( $name, json_encode( $response ) );
        return $name;
    }
}

3b.php

$fileName = ( new \calderawp\WordPressPlugin\Jobs\PostSaveJob( $postId ) )->handle();

3c.php

class PostSaveJobTest extends TestCase {
    public function testSaves(){
        $postName = 'foo';
        $postId = wp_insert_post( ['post_content' => 'fff', 'post_type' => 'post', 'post_status' => 'publish', 'post_name' => $postName ] );
        $fileName = ( new \calderawp\WordPressPlugin\Jobs\PostSaveJob( $postId ) )->handle();
        $this->assertTrue( file_exists( $fileName ) );
    }
}

4.php

add_action( 'save_post', function( $postId ) {
    wp_queue()->push( new PostSaveJob( $postId ) );
});

4b.php

add_action( 'save_post', function( $postId ) {
    wp_queue()->push( new PostSaveJob( $postId ), 600 );
});

5.php

add_action( 'plugins_loaded', function(){
    wp_queue()->cron();
});

6.php

add_filter( 'wp_queue_default_connection', function() {
	  return 'sync';
} );

7.php

function workQueue( $jobs = 10, $attemptsPerJob = 3 ){
    //Get a properly configured queue
    $queue = wp_queue();
    //Get the worker for the queue
    $worker = $queue->worker($attemptsPerJob);

    //Loop through jobs in queue until we've done the number passed in $jobs
    // or no jobs left to run
    $totalJobs = 0;
    while( $worker->process() && $jobs <= $totalJobs ){
        $totalJobs++;
    }
    //Work the queue and
    return rest_ensure_response( [ 'totalJobs' => $totalJobs ] );

}

add_action( 'rest_api_init', function(){
    register_rest_route( 'hi-roy', 'queue/run', [
        'methods' => ['POST', 'GET' ],
        'callback' => 'workQueue'
    ]);
});

Working through the implementation of your article on https://torquemag.io/2019/01/using-the-wp-queue-to-copy-rest-api-data-to-files/ and I'm wondering...shouldn't line 10 of 7.php to be:
while( $worker->process() && $totalJobs < $jobs ){
instead of:
while( $worker->process() && $jobs <= $totalJobs ){

...swapping the order of comparison allows jobs to actually run
...changing the operator limits the number of jobs run to the $jobs rather than $jobs+1

I had to rewrite the workQueue function as the register_rest_route() method passes a WP_REST_Request to the callback function. This may have changed since the tutorial was written. The $jobs and $attemptsPerJob can be passed as parameters in the request.

I also had changed the original line 10 to be:
while( $jobs <= $totalJobs && $worker->process() ){
...this stops the while loop at the correct number of jobs to be processed rather than $jobs+1

I forked this gist and updated it there if there is any value in sharing/updating