I hope this is not the case, since I don't believe this

That was poorly worded. I'd already updated the grandparent before you posted this; hopefully the revised version will be clearer.

Why? The other three premises do not imply that I am a Boltzmann brain. They only imply that model X predicts I'm a Boltzmann brain. That doesn't conflict with the second premise.

I was talking about my formulation of the problem, not yours. Assuming you're not a Boltzmann brain does lead to a contradiction with one of my premises, specifically the one about invalid observations.