The key word is "access." In order to attack via cyber mechanisms, the hacker must be able to reach the targeted system via some external means. To use a stupid example, if I buy a computer but never connect it to the internet, and I never put any third-party software or files on there (i.e. the only software or data comes from me personally) then my computer can't be the target of a cyber attack because there's no way for a hacker to access my system. I could even have a local wired network with multiple computers but if none of them have external access, then they can't be targeted via cyber means.
If there's no external access, then the only way to deliver a cyber attack to that system is by physically getting a human being into my facility, breaking all my username and password protections, and successfully uploading a file that I didn't create. While that's often seen in movies, it's incredibly tough to do in reality.
The one caveat for external human access is insider threats. The larger the system/network, the greater the risk that an authorized system user could inject malicious code. A disaffected user, or a vulnerable user susceptible to bribes (financial or otherwise), could provide access to a foreign nation, which is why Governments put a lot of time in layered mechanisms to prevent it such as human security (background checks etc), leadership/management (ensuring staff aren't disaffected/vulnerable), access control (limiting users to specific functions so a general user can't upload malicious code), and monitoring (ensuring staff aren't alone when they access a system, and ensuring all actions are logged and alerted in real time).
Cyber isn't a silver bullet. It takes a lot of time and hard work to develop access...and after all that work, access can disappear in the blink of an eye if the vulnerability in the target system is identified and closed off (e.g. regular software updates to close security holes, firing (or otherwise removing) possible insider threats, etc).
Bottom line...if the nuclear computer control systems do not have external access, and the staff are patriotic, loyal and largely incorrupt, then the systems are pretty much unhackable.