I just tested this article in AEM 6.3 and it is working perfectly fine.

Please note you have:
1. change name of taskPriority to country
2. Change sling:resourceType type to granite/ui/components/coral/foundation/form/select
3. Delete datasource node
4. Create items node and add properties as mentioned int he articles.
5. [Important] Create a clientlibs node under /apps/myDropDown63/components with node type as cq:ClientLibraryFolder and add categories and dependencies as mentioned in the article
Create a js.txt file under /apps/myDropDown63/components/clientlibs having text as demo.js
Create a demo.js under /apps/myDropDown63/components/clientlibs having text as JS code mentioned in the article.
smacdonald2008 I would request you to add final package (after the intended changes, so that member/reader can immediately work on demo).
-Kautuk