I doubt it.

Keep in mind this will only work for non-court-gag-ordered instances. If the US subpoenas Apple about an individual they won't be allowed to notify them.

I have no idea how this applies to other countries.

I think this is more like: "We noticed unusual API usage and we don't have a gag order so whatever it is, it's not likely to be good"

The methods of detecting such attacks are not at all similar to a government requesting data which contains the non disclosure clause.

Apple doesn’t need to know the source of the attack to issue the warning, and if the attacker is competent Apple likely wouldn’t know the source, such that a gag would not apply.

To be fair, a subpoena isn't a cyberattack. But yes, this will be mostly of value of people being targeted by governments that are not the USA or best buddies with the USA.

tl;dr: Apple will notify us as long as the attacking state isn't the US - which it very often is.

It's rare that programmes like PRISM surface publicly. I don't see how Apple would gather top secret intel on national surveillance programmes on their own, so there is a good chance they aren't even aware.

In the case of Google, the NSA was reading their unencrypted replication traffic as it moved between data centers.

I don't see how Google could have been aware that this was happening, although they certainly could have known it was theoretically possible.