How does Adobe know the rights to an image belong to me if I use a pen name?
There is no need. If there is a complaint you need to prove that you licensed the asset. That's all.
And is Medium story deemed as commercial use?
A blog is considered editorial use, so you need to attribute as you did (author/stock.adobe.com --- according to the licence terms). Your use seems to be covered by the licence you acquired.
Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788 (Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).
ABAMBO | Hard- and Software Engineer | Photographer