The documentation you mentioned have a part describing internal strings and the rules applied:

This process is in fact a little bit more complex than this. If you make use of an interned string out of a request processing, that string will be interned for sure. However, if you make use of an interned string as PHP is treating a request, then this string will only get interned for the current request, and will get cleared after that. All this is valid if you don’t use the opcache extension, something you shouldn’t do : use it.

When using the opcache extension, if you make use of an interned string out of a request processing, that string will be interned for sure and will also be shared to every PHP process or thread that will be spawned by you parallelism layer. Also, if you make use of an interned string as PHP is treating a request, this string will also get interned by opcache itself, and shared to every PHP process or thread that will be spawned by you parallelism layer.