Is this the correct use of "opt-in?"

To me, having things "opt-in" means they're off and you can turn them on if you want.

If it's "opt-out" it's automatically on, and you can turn it off.

Likewise, I think the title is literally of opposite what is actually happening.

I think they mean 'Enabled by default'

You are correct. The headline author likely meant "opted in by default" or "enabled by default."