That contradicts one of the assumptions in the thought experiment. You're establishing qualia as a physical property; in that case, "what it feels like to see red" is amongst the things Mary knows about, by hypothesis.
Also, if it just comes down to activating those neurons, then Mary knows that too and can perform an experiment to activate those neurons without having a 'red thing' in front of her, using her incredible superhuman intelligence and resources.
I am not establishing qualia as physical properties of brain's activity, I think of them as descriptions of specific neural activity in the terms of human's self-model. And limitations of that self-model (it's not sufficiently detailed to refer to individual neurons) don't allow to establish unambiguous correspondence between physical description of brain and self-model description of brain within that self-model.
...Mary knows that too and can perform an experiment to activate those neurons without having a 'red thing' in front of her, using her incredible
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