It seems to me that the author completely missed Medium's import tool in his/her "mirror your blog" section. The tool is specifically made to prevent SEO issues when syndicating content from your own blog.
The key part of this functionality is, "Imported stories will automatically apply a canonical URL referencing the original source material." Taken from the content of that page.
https://help.medium.com/hc/en-us/articles/214550207-Import-a...
There's no SEO risk as the author alludes to. This is Medium's tool for dealing with that.