My understanding is that you cannot transfer STEEM from your wallet on steemit to the exchanges, so you cannot appear as seller with newly arrived STEEM on the markets. So there is no "new" direct supply, only as you say, if you sell BTC or shitcoin for STEEM and sell that a bit later.
I am afraid when the wallet transfers will be enabled again in a few days, we will see a drop in price as new STEEM supply will arrive to exhanges. I just hope there will be enough buyers to balance that extra supply.