If you read the first sentence of the paragraph and last sentence of the paragraph as well:

"The only way that I can see [a way to peg SBD] would be to disallow trading on external markets. In other words, Steem is traded on exchanges, while SBD can be sold only on steemit for exactly $1. (And the conversion between steem and SBD also only occurs only on steemit)."

The fact that SBD is trading on other exchanges, like Poloniex, is why its price cannot be fixed at $1.