If what you say were true, then any speculator selling high and buying low would be "supporting" price (bitcoin or no bitcoin) which is obviously not the case, since for every deal there are two parties, one selling and the other buying. The only direct way to support currency would be to buy it when prices are falling...
I think majority of the speculators can not sell high and buy low accurately. My example just showed that if they successfully did that, their action will support the exchange rate without incur a loss. And there are some experienced institutional traders, they are usually the driven power of price development
Unlike gold, bitcoin's supply is limited,
for each coin you buy or hold, you reduce the sell pressure on the market by 1 coin permanently. It does not take too much fiat money to drain out the coin supply on market at today's exchange rate
On the contrary, if you buy and hold coins, this diminishes liquidity and thereby increases volatility. How could you possibly stabilize the exchange rate through increasing volatility? It is liquidity that supports exchange rate, leveling off speculative component from it...