Centralization vs Decentralization is a similar concept to closed source vs open source. Just because the code is open source doesn't mean that people are going to download the source code and compile it themselves. The option to fork the codebase is also there. Neither of those options are possible for a closed source project. And to your point, the fact that a project is open source might not "actually have any effect at all". Doesn't mean the concept is inherently bad.
This almost makes sense, except that you have to take into the relative value of that amount, in particular relative to one's total net worth. If I want to protect $100, and that is all the money I have, I'll worry about security a lot. If I have $1,000,000, protecting $100 is not a priority.
The general assumption is that people put all of their money in one place, which is not true for anyone I know -- poor, middle class, or rich. Everyone has cash, bank accounts, various physical valuables, stock, etc.
The point that Moxie was making at the time was that, pragmatically, most users would end up not doing this. Just as most people share their thoughts on Facebook or Twitter and not on their own blogs, and if they do have their own blogs they don't run their own server, and if they do it's a virtual machine on Amazon's hardware.
So something can be theoretically capable of enabling enormous decentralisation but actually not have that effect at all.