Participation was limited because US residents-- by far the largest population of cryptocurrency users-- were prohibited from participating (though of course some did). [Edit: A comment below claims otherwise, I don't actually see support for that in my source, and my own correspondence from the time states that it wasn't. I'm not sure if something changed around the launch, or if I'm somehow mistaken.]
I dunno anything about winklevoss speculation, people often include funds held in custody as if they're owned by the custodian and reach weird conclusions. It's easy to list ethereum accounts that hold >=1% of all ethereum: 0x281055afc982d96fab65b3a49cac8b878184cb16, 0x6f46cf5569aefa1acc1009290c8e043747172d89, 0x90e63c3d53e0ea496845b7a03ec7548b70014a91, 0x53d284357ec70ce289d6d64134dfac8e511c8a3d, 0xab7c74abc0c4d48d1bdad5dcb26153fc8780f83e, 0xfe9e8709d3215310075d67e3ed32a380ccf451c8; while the largest Bitcoin address is under 1%. (Of course, parties can have multiple addresses, and as mentioned some parties hold coins for many other people)
Moreover, regardless of the distribution Bitcoin had no premine: Everyone who has Bitcoin received it in a open process available to the whole world with no particular privilege other than knowing and caring about it when it was almost worthless before other people did. (well, I suppose except for those people who stole theirs :)).
We don't actually know how many Bitcoin Satoshi-- if he's still alive-- owns: there are guesses based on assuming various earlier miners might have been him, but they're guesses. The 1M claim specifically is flat out false: it comes from counting up every coin mined during the first year that was still on spent at the time the figure started circulating a few years ago. It includes many coins known to have been mined by other people, including likely myself.