I don't see how it can be too complicated. When miners sell their stash of coins, they are just like any other Bitcoin sellers, and the effect they have on the price is in no way different than it would be if someone else had sold the same coins. The only difference is that they sell newly minted coins. The price changing effect is absolutely the same. If what you say were true, it would mean that the bitcoins that miners produce and sell somehow avoid affecting the price of Bitcoin...
In short, you shouldn't consider the miner's reward in isolation, taken per se, but you should look at the supply of new coins which enters the market and where the reward plays a secondary role as a factor in quantifying the amount of this supply