We have a short snippet on this page under the heading “The meta factory has been deprecated”. Basically that tutorial you linked to is now defunct. You can now follow our standard themer’s guide as outlined in that link.
If you have any feedback for us on this change or on our themer’s guide please let us know. We deliberated this change a lot, but ultimately decided that switching from the meta Walker was going to be the easiest to modify solution going forward. That is why it is being depreciated.
Does that make sense? Will that work for you? Please let me know. Cheers!