There are plenty of other interesting features, such as disallowing conditional branching on the contents of secrets, all the way down to enforcing Spectre and Meltdown mitigations around secrets without necessarily globally taking that performance hit on sensitive and non-sensitive data alike.
Spectre/Meltdown mitigation is a great example of the latter. The compiler can be careful to emit instructions that don't permit speculative branching, or disable other optimizations that might expose a security risk. Libraries don't typically get the kind of access that would permit them to do that, and it's not clear to me that a language that does permit such access would be appropriate for safety- or security-critical applications.
As for why if statements are a security risk, they give a tidy little example of that at the bottom of their overview page, under the heading, "Can you see the security flaw in this code?" I can't say I understand it terribly well. I personally take that as a sign that, while I can speculate about why they're doing things this way, and even be arrogant enough to do so in public on the Internet, I'm very, very, very far from being qualified to suggest that their basic approach is wrong. Chesterton's Fence sometimes strikes me as being overblown, but Chesterton's Safety Belt is no joke.