I didn't think they could do anything with the peg will the price of SBD is greater than $1. And they only try to 'peg' it when it falls below $1.
I might be wrong because I'm learning too.

Also, I think the price tends towards parity because external investors are confused which has the most utlity and therefore if one is low they consider it to be cheap. Or if one is high they consider it to be overpriced and take the profits.