Heres what I think on the whole PBOC keeping BTC below 1000 US dollars conspiracy. The higher the price of BTC is, the easier it is for money launderers to use it. Think of this, if you want to launder 1 million US dollars, would it be easier for you to launder via BTC if it was at 10 dollars or at 10,000 dollars?
I don't think it actually matters
At least, as long as we talk about the same amount of fiat money (US dollars, euro, yuans, etc). Really, 10,000 dollars will remain 10,000 dollars no matter what Bitcoin price might be. In other words, you don't need to launder bitcoins themselves since they are of no particular use on their own. You can't do much with them (if only gamble them away). You can multiply the coins through trading or other investments but to use them in real life you will still have to convert your bitcoins to fiat first. And this is where the size begins to matter
Of course it does matter. The value of your fiat would depend on it. If you want to launder millions, which in most cases the amounts are, a higher BTC price will not cause any spikes in volatility as compared to when its lower. Pumping millions in and then getting them out will abruptly make the price go up and down.