So:.why think memory and computation capacity isn't important?

So memory is important, but it scales with the mass and that usually scales with volume, so there is a tradeoff. And computational capacity is actually not directly related to size, its more related to energy. But of course you can only pack so much energy into a small region before it melts.

The data centre that will be needed to immerse 7 billion humans in VR is going to be huge - and why stop there?

Yeah - I think the size argument is more against a single big global brain. But sure data centers with huge numbers of AI's eventually - makes sense.

The 22 milliseconds it takes light to get from one side of the Earth to the other is tiny - light speed delays are a relatively minor issue for large brains.

Hmm 22 milliseconds? Light travels a little slower through fiber and there are always delays. But regardless the bigger problem is you are assuming slow human thoughtrate - 100hz. If you want to think at the limits of silicon and get thousands or millions of times accelerated, then suddenly the subjective speed of light becomes very slow indeed.