This was in the January 20th (2021) "Top Edit" Newsletter to "creators" (which a creator or writer receives if she/he has ticked the following), under Settings. 😃

The “Top Edit” Newsletter replaced the “Medium Partner Program” Newsletter some time ago. When I started with Medium in November 2018 they had a monthly “Medium Partner Program” newsletter only for those in the Medium Partner Program (MPP). Medium changed the title to “Medium Writers Newsletter” in 2020 and in 2021 they now call it “The Top Edit”.
Anyhow I think that the message you show at the top of your article refers to using the Medium “app” on a mobile phone or mobile device. I found that when I go to a previously published story that I can see Share.
Clicking on “Share” allows me to email that one story to one or more people.
Meaning I can quickly add one Email Address to the Recipients and click Send and it sends a link to the story. I guess I could add in many email addresses or try to select an Email List Group of subscribers if I could work out how to do that using a small phone screen and by configuring an Email Distribution List via my Email facility.
I think when the message says “share with your subscribers” it just means share this way with one or more people, as you configure it. They should have qualified this “sharing” is only for “apps”.
