Thanks @mdonnelly that is helpful but also a little confusing as I am not sure what step 6 is trying to say. It indicated that you need to publish a map with the symbology but not sure what that really means. Is it that you publish a series of features, one for each symbol or just have at least one of the symbols in the map so when published the symbology is contained in the uploaded file. If you have contacts with the article author it would be great to have that clarified.
Yes, my understanding is that you would include features for each custom symbol that you wanted to host. This is supported by what it says in step 7:
The reliance on the hosted feature service is removed once you download the symbols and host them on a web server.
Unfortunately, I do not know who the author of this article is but if you get really stuck you can contact Esri support and they should be able to clarify any points for you.