A couple of things are puzzling me:
- I thought UML templates were only included with the Professional edition.
- You have the Standard edition of Visio and yet your "Featured" template page includes two other templates that I know for a fact are only in the Pro edition: Detailed Network Diagram and Data Flow Diagram.
For #1, I will check with the Visio team at Microsoft.
For #2, I spun up a virtual machine so I could look at a copy of Visio 2016 Standard, and sure enough, I also see templates on the Featured page that I know are only in the Professional edition. I will check with the Visio team about this as well, because
it doesn't make a lot of sense that you would see templates that you can't use. Unfortunately, I think this is what's happening here: the UML template is only available in the Pro edition, despite its thumbnail appearing on your Featured page, which is why
you can't locate the stencils -- they are not part of the Standard build.
BTW, I definitely see "Visio Standard 2016" at the bottom of the article you included with your initial question... if #1 above is correct then the article text is wrong.
Stay tuned... we'll get to the bottom of this, even if the answer is what you don't want to hear, namely that there are no UML diagrams in the Standard edition.