It's also theoretically possible for hackers to steal Apple's private key from their servers, in the exact same way. As far as I see, there's no change to the threat model by Apple making a new software version, and signing it using the same process they use to sign other versions of iOS. It's useless if not signed, so the real worry is someone having the ability to sign it, and that applies exactly the same if Apple signs iOS or FBiOS.
This suggests the authors don't understand the technology well enough to know this.
edit: this is assuming the software is locked to a specific device. So the signing doesn't matter to any other device. If the software worked on all devices, then a leak of a signed version would be problematic. Although even if they couldn't lock it to device, they could make it only work for a short time, say a week, so if that signed version leaks later it would have no effects (I'm not sure if you can change the clock on a phone without unlocking it though).