I have no strong opinions on this, I don’t need the @ tags for people myself. My only point is that the @ symbol is already used in conjunction with markdown for citations.
There might be people who want tags for people AND citations, so using this notation would create conflicts for them.
I’ll just point out that this are second class for you, but you can check the Zotero integrations thread and see how many people consider citations a first-class use, and also the requests for pandoc. My only point is: the use of @ would cause conflicts for the use cases other people want too, might be a good thing to consider. Dependencies and conflicts between plugins might get messy otherwise.