Bio neurons produce action potentials by depolarizing when reaching a threshold voltage, they're certainly very different from matrix multiplication operations.
The thought experiment is that a group of bio neurons can perform the same simple mathematics primitive that artificial neural nets use (demonstration: you can do a simple fused multiply-add in your head). So there is in principle a way of hard-wiring them that will approximate the function that a digital neural net based on matrix multiplication computes
Then it's a matter of taking your digital tokens and converting them to the encoding you use for your bio-neuron circuit, which is maybe a concentration of calcium in presynaptic neurons, a voltage, or whatever else is more convenient.
You don't directly take a brain and feed it tokens somehow. You emulate a digital circuit on squishy substrate in an extremely inefficient way, such that it runs the exact same program/does the exact same computation with a substrate made of biology instead of transistors. You make an injective function from boolean functions to cells and proteins, just to belabor the point that in principle GPT-3 can talk like GPT-3 and say it feels pain, without a single transistor being involved.
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