It’s not an advantage, in practice. Writing exploits against iOS is a very scarce skill, and the people who can do this might be slightly more productive if aided by the source, but the reverse isn’t true. Having the source doesn’t teach you anything about finding and exploiting these bugs.

It might seem logical to those unfamiliar with how these hacks work, but consider that such hacks do not depend on the secrecy of the design. Also consider that Apple hires regular software and hardware engineers, who do their best to design a system, and Apple then hires hackers (both internally, as well as external consultants) to find weaknesses in their designs. These weaknesses are then fixed before the product ships, meaning even those who were paid to break it no longer know how. This alone should tell you that people who know the system intimately are not the ones who understand how to break it.

Put another way, if I need to make this product, and there are two candidates I can hire, one person who wrote the software and one who knows nothing about the software but is demonstrably skilled at finding exploits in similar systems, I’ll take the latter in a heartbeat.