His point is that mining is the only source of inflation in the system. If the network hashrate increases a lot compared to difficulty, let's say 12 blocks an hour instead of 6, then for the time, until the difficulty adjusts, the supply of Bitcoin will increase.
The point about mining being only 20% of the total market is irrelevant. It is the only inflationary component to the market, so it is the only one that naturally reduces price.
I think you are wrong. Bitcoin supply isn't all 6M bitcoins that have been minted, but only those that are used for trading. From the market point of view, bitcoins that have been minted but never used for trade, might as well not exist. I suspect that a lot of that kind of coins are entering the market for the first time.