Both are right.
It works like this: when using Halion as a multitimbral synthesizer, you will probably be better off with Multicore on.
But if, as recommended, using one instance of Halion per timbre, you are better off with Multicore off - because the thread scheduling can then be done by Cubase, which has far more information about the system state than one instance of a VSTi.
At least this is how I understand things (and I’m a software developer in my daytime job as well).