We use campaigns for more than just marketing/advertising... like this use case... any push notifications that are driven by speakable to send people to our site from news briefs are captured via the appended UTMs to indicate these sources (the UTMs are captured in Adobe and we have a report to show at least incoming traffic from those particular sources).... this is the only tracking I have so far when it comes to Smart Speaker content....
Our News Briefs aren't paid... there's no advertising around them...on a smart speaker, people just say "[trigger] give me news for [location]"... If we have relevant content that comes up for those users, they are read by the smart speaker (we can't track that though... it's outside the reachable tracking area - like Google Search Results). However, the smart speaker will ask if people want the content pushed to their phones... if they do, we have the campaigns as part of the push notification link so that we can track the traffic from that source.
As I said, this is only one small part of the overall....
The article you referenced in your original post looks more like how to add tracking to a smart device app... so that you can actually track the use of certain commands within your own app. Which based on your "if someone visits site via Siri or Alexa" doesn't sound like that documentation is what you are looking for (but maybe I am mis-reading your question).
This is a such a new thing, it's going to take time for us all to figure stuff out... if you do find another piece of the puzzle, I encourage you to share it here (or in a discussion) so that you can help other people in the same situation. 🙂