It's simple enough. If someone seriously argues that under atheism all our thoughts would be are just brain fizz, then how can they trust what they think? Reason being, atheism might be true and all their thoughts and conclusions are just their brain fizzing. How could they tell if their thoughts were coherent or just brain fizz? There is no way out of this self constructed trap.I don't understand your objections.
The brain fizz argument is fallacious for two reasons, one as I've said above, the other, I've never seen it justified, only asserted.You already believe you are material, dirt and mostly water.
Your thoughts are electrical stimuli.
What other 'mystery' there be is no mystery for you, it's just material arranged in a certain configuration, with temporary stimuli animation.
Pretty simple really.
If you truly believe something substantially different, state so.
Otherwise live with the above, it's all you got by your own definition of life, the universe, and everything.