@paulag, I've written two new posts on Steemit where the original appeared elsewhere first. In both cases, the re-publication happened mere minutes later than the original. That's significant as it is unlikely that the original was indexed yet. Also, in both cases, I used the canonical link tag to point to the original article. Thirdly, my title on Steemit was slightly different than my title on Narrative. And fourthly, Narrative is only a few months old so Steemit has a significant head start on domain age. Here's what I found.

In one case, Google ranked the Narrative version on top. In the other, the Steemit version is on top. Still inconclusive.

I'm using the Brave browser with DuckDuckGo as my default search engine. But I conducted my searches using Google. I then conducted the same searches with Microsoft Edge, which I never use and where I am not logged into my Google account. Same results.

It's possible that some of Steemit's posts are outranking canonical content posted elsewhere, but it's also possible that Google is the reason why and not necessarily Steemit's attempt to control the search results. Here are the links to my Steemit posts in case you want to run your own experiment.

Story 1 and Story 2

I'm still testing.