I think the difference lies in two things:

• You can share an article from a user of a different instance. In this case, your instance will have to look up the rel="author" tag and check whether the URL is a fediverse instance. I'm not sure whether this is scalable as compared to a tag that directly indicates that the author is on the fediverse. Imagining a scenario where there are 100, 1000, 10,000, or 100,000 instances on different versions.

• The tag is to promote that the author is on the fediverse. If the rel="author" tag points to twitter for example, maybe Eugen Rochko + team didn't want a post on the fediverse to link to twitter.

These are my thoughts and idk if they're valid. But I think just reusing the rel="author" isn't the most elegant solution.

I know that mastodon already uses rel="me" for link verification (I use it on mu website + my mastodon account), but that's a different purpose - that's more for verification. There's still no way of guaranteeing that the rel="author" tag points to a fediverse account. You're putting the onus on the mastodon instance.