The problem is that if you don't believe no matter how hard you try, you'll trip up. You can't make yourself or someone else believe. Cos that what the human brain does - you can't hold two thoughts in your head at the same time and get it right every time. The cognitive dissonance is impossible to over ride.
This is true. As you say, we saw it in the Sandie Peggie case, but I think it's also been true of every single other tribunal, eg Allison Bailey, Jo Phoenix etc, that there has been a point where either a witness or a barrister has misgendered someone. Usually it's someone on the trans activist side, ironically, who trips up.
Barracker's famous 'Pronouns are Rohypnol' essay explains the phenomenon very well.
I agree with others that it's a shame that the claimant had to use her disability as an excuse for misgendering. People ought to be able to say, clearly and plainly, "I refuse to lie". But the Forstater judgement made clear that, while believing that humans can't change sex was a protected belief, it still wasn't acceptable to deliberately misgender your work colleagues.