

You can add captions to images inside text posts - those are used for alt text. You can’t add alt text to image posts though.
Trust me, I would know.
Incidentally, they’re automatically adding the subreddit and post title together for the image’s alt text, which still doesn’t include OP’s name.
Right, as an instance admin, I’m very confident I can avoid Facebook’s mistakes. I don’t have the same motivations.