Apple has done plenty to lose my trust, and very little to build it. But that's not really the subject at hand, though I do see where word choice is misleading here.

You just brought up a better word: "liability". I'll go one step further: "attack surface".

When it comes to security in software, we don't need to work with many unknowns. The unknowns we do work with are the attack surface. By presenting a greater domain of unknown behavior, closed source software effectively presents me (the user) a larger attack surface. Sure, I could trust that the extra attack surface is actually covered; but I can't know. With open source, I don't have to trust, because I can know instead.

If I am to choose between open and closed source software, then I am choosing between knowledge and trust. That is a completely different position than choosing between closed and closed: trust vs. trust. So long as any securely-designed open-source messaging app exists, iMessage is at a disadvantage in end-user security. Even if Apple can know for certain that iMessage's attack surface is not larger than an open-source alternative, we the users can't. Closed source software will always present a higher demand for trust.