I disagree with you there. Markets are interconnected. When you sell STEEM, it doesn't really matter whether you are selling it for SBD, USD, BTC, or bartering it for camel feet. Selling is selling, and it drives the price down. Demand to buy and hold an asset is what drives the price of the asset up.
BTW, pre-July 4, Dan made the argument that people buying STEEM on the internal market in exchange for SBD would drive the price of STEEM up (I partially agreed). You can't have it both ways.
Yeah, as I've thought about this more I'm beginning to see that. In fact, those sell walls on the internal market are partially maintaining the dollar peg of SBD. If the sell walls are high enough, then the STEEM price partially becomes an indicator of how many people are trying to get out of SBD.