If I were a trader and saw that margin calls happen regularly, I would place my sell orders 2% above the price at which the least collateralized short position is margin called, for an instant 2% profit. If I wanted to buy CNY on the DEX I would place my order 2% above that price, because otherwise I have no chance of getting it filled.
Thus, margin calls and feed price and MSSR all interact with each other. The DEX price is driven by margin calls, which are in turn driven by the feed price and MSSR.
If you reduce the MSSR to 1% the premium will also go down to 1%. Does changing MSSR change the "fair" price of bitCNY? I think not, but according to your logic the fair price would then be 1% higher than it is now, not 2%.
On an external exchange you don't have that effect, because the external exchange is not influenced by margin calls in the way that the DEX is. The result is that the external trade price is much more natural than the DEX price, and so can more reasonably be called a "fair" price.