This argument implies there's an easy way for you to perform the reproducible builds on iOS, but it's quite involved and requires a jailbroken iPhone. Overall this is more a limitation of apple and not signal.

Even if you were able to perform a reproducible build of Signal on a jailbroken iPhone, there's no way to confirm that the stock iOS Signal app will match, or has a backdoor that got added in a supply chain attack that only is delivered to non jailbroken phones. You could use a jailbroken iOS device, but then it could be lagging behind updates and be even more vulnerable from zero days.

The real pressure here should be on Apple to provide a way to verify a build of an open source app matches what is being installed via the app store, but for some reason this is being framed as a Signal issue, which is disingenuous.