users hand over over ownership to reddit the moment you post

Not ownership. Just permission to copy and distribute freely. Which basically is necessary to run a service like this, where user-submitted content is displayed.

And since there's no such clause on Lemmy, they'd have to ask the actual authors of the comments for permission instead?

It's more of a fuzzy area, but simply by posting on a federated service you're agreeing to let that service copy and display your comments, and sync with other servers/instances to copy and display your comments to their users. It's baked into the protocol, that your content will be copied automatically all over the internet.

Does that imply a license to let software be run on that text? Does it matter what the software does with it, like display the content in a third party Mobile app? What about when it engages in text to speech or braille conversion for accessibility? Or index the page for a search engine? Does AI training make any difference at that point?

The fact is, these services have APIs, and the APIs allow for the efficient copying and ingest of the user-created information, with metadata about it, at scale. From a technical perspective obviously scraping is easy. But from a copyright perspective submitting your content into that technical reality is implicit permission to copy, maybe even for things like AI training.