The main purpose of the event is for active Bitcoin Core contributors to meet and accelerate completion of many outstanding projects. Meeting in person to code together is very productive. This is a "hackathon" rather than a conference which focuses on tangible work.
The "invite only" strategy aims to reduce developer distraction and allows to focus on the main goal: specifically work together on Bitcoin Core and Lightning. For this reason, only regular active team members and contributors to Bitcoin Core have been invited, along with Lightning developers because Lightning is an important part of Bitcoin Core's capacity increase roadmap.
There won't be any video or audio streaming but developers will continue to use #bitcoin-core-dev IRC chat as normal.