In short, WordPress.com would need to support at least two of the Indieweb building blocks: Webmention and full Microformats 2. See all the building blocks at the first link.
In order for WordPress.com to be a turnkey solution, it’ll need to support these things out of the box, and make it as simple as checking some boxes, or better yet, turn them all on for everybody by stealth.
part of this involves themes, and for the time being users either have to install a plugin like MF2 from the WordPress repository which will try to programmatically add Microformats 2 to a theme, or choose a theme that has full Microformats 2 support already baked in, or manually add them to a theme themselves.
I’m not saying WordPress.com couldn’t do this, (I’d love it if they did, and if they became a turnkey solution for people who want to join the Indieweb), but I don’t see that happening any time soon.