> The price hasn't been a constant $8200 during the issuance of those tethers, it was lower earlier.

The same holds for the bitcoins bought with dollars at Mtgox.

But to end the discussion:

  Dec 31, 2013: Marketcap 10  bn, Vol 34 mn
  Today       : Marketcap 240 bn, Vol  8 bn

So 600 mn had a much bigger impact (20x daily volume back then) than they'd have today (0.075x daily volume).