That means that they believe there's more value in having those exploits, not necessarily that they're harder to find. Though I imagine it's at least a little bit harder because iPhones are closed source. That does also mean that if Apple doesn't discover the vulnerability, it's unlikely to get a patch. With Android, these security issues are found more easily, but also patched much faster. Which I suppose makes an Android vulnerability also worth less, since you get more limited use out of it.

Iam on grspheneOS