Apple is digging itself into a hole.
In light of the correct legal interpretation of their words, being only the specific letters, we can see that your interpretation is incorrect.
> They know of a lot of attack attempts
No, their statement says nothing about attack attempts.
> so far they have no reason to believe any were successful
No, their statement says nothing about their belief, only their explicit knowledge. Their statement says nothing about their investigation practices or whether they even attempted to investigate and learn about attacks. Their statement says nothing about non-mercenary attacks.
Their statement is technically correct as long as any successful attacks they know about are not explicitly known to be committed by mercenarys.
That's a good point. The best way not to know about any successful attacks is not to know about any of them. I also can definitively state that I'm not aware of any successful attacks, but for obvious reasons this is a basically meaningless statement. Without more data, it's not clear how meaningful the statement they gave is, and while it probably is more meaningful than mine, it doesn't make sense to jump from what they said to "there have definitively been no successful attacks" based on it.
> No, their statement says nothing about attack attempts.
Exactly, they're keeping the statement brief and correct. They have sent multiple batches of notifications to users on previous attacks.
The statement is clear, covers their primary use case for the product, and I'm sure is legally sound. You're grasping at straws trying to think up ways they can be lying to you. I would be very surprised if you ever have used their lockdown mode with any actual cause.
At risk of stating the obvious, isn't success "hacked it and no one ever found out (at the time)"? By definition, Apple could probably only be aware of unsuccessful attacks. Though that's not guaranteed either, considering all the myriad failure modes that there must be.