Agreed, ML will be able to extract signals from this data, no question. The electrodes really are the limiting factor I think, because you're going to need millions placed all over the brain and deep inside too for a truly useful implant.
For sure, increasing # of electrodes + bandwidth must be on the roadmap.
You may not need to go as deep and broad as you believe. Most processing happens on the cortex anyway, the interior (white matter) is pretty much interconnection. Also, the action of one neuron could trigger thousands of others in quite remote regions, so you could probably get a decent picture of what's happening without having to cover it all.
I was honestly quite impressed by "the pig demo", I wouldn't have guessed that would be possible with the current state of their tech.