Yes this is right. The trustless system we talk here has nothing to do with two-generals problem. Communications have no problem, the problem is that how to prevent any node from doing bad things (i.e. steal coins).
Actually no, the "guarantor" is only involved in the act of sending coins, which is a "communication" in the context of the Two General's problem.
The "guarantor" is being trusted to do arbitration between the sender and the mixer. Therefore, given the nature of 2-of-3 multisig transactions, the guarantor and the mixer can sign the transaction, and then refuse to sign the cancellation transaction, leaving the sender out of luck and out of funds.
Even worse - the workflow is based on the txid and verifying the txid. Have we not learnt by now that the txid can change? How do you people not understand that this was the very thing that mtgox blamed for their destruction?
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The assumption is max 1 bad guy in the 3 party scenario. If the bad guy is guarantor, then he does not have a say as sender/mixer will sign and complete the multisig tx. If the mixer is the bad guy, then guarantor will make the judgment, I don't see any issues there.