This should work just fine, assuming you used urls like "/yyyy/mm/dd/title-slug-here", and that you have Composer installed. Put these files in a directory containing only your Wordpress xml export file. Open "convert.php" and change line 3 to match your domain name. Then run these commands on the commandline:
composer update
php convert.php
You should see a message like: Exported 6 pages and 792 posts from /websites/wp2secondcrack/derekmartinca.wordpress.2017-03-16.xml
That's it - you should now have year/month/day directories full of markdown files.