It's trained to complete text, but it does so by constructing internal circuitry during training. We don't have enough transparency into that circuitry or the human brain's to positively assert that it doesn't assume.

But I'd wager it's there; assuming is not a particularly impressive or computationally intense operation. There's a tendency to bundle all of human consciousness into the definitions of our cognitive components, but I would argue that, eg., a branch predictor is meeting the bar for any sane definition of 'assume'.