Are you serious ?

First of all, with sidechains, we're talking about the difference between one currency and another. A sidechain token and a mainchain token are not capable of mutual substitution because they are entirely distinguishable (having different identities). For example, a bitcoin cannot pass as a confidential transactions coin since it doesn't remotely share its properties and vica versa.

In Dash, all tokens are on the same blockchain - whether they're pre-mixed or not.

Secondly, in money (as opposed to cyphering systems used by spy networks), untraceability is maximised by the aggregation of continual mixing and exchange of tokens on the financial network, not by hiding the network transactions. Dash follows this paradigm because it is a cash paradigm (not a credit one based around bookkeeping notions of privacy) and maximises transparency at the same time.

Thats the beauty of it.