Supposing for the sake of argument that the majority of the board privately believe that Sam is a snake, but there's no smoking gun. At this point you're saying they have 2 choices:
1. Let him leave with 60% of the staff. 40% stay at OpenAI
2. Let him win this fight, he's back at OpenAI with de facto 100% control, no check on his power over the AGI that OpenAI is supposedly creating
I'd rather have a snake lead 60% of the staff than 100% of the staff.
On the other hand, if he leaves with 60%, maybe he decides he needs revenge at all costs, and his new AI org doesn't even pretend to be in it for humanity.
Tough choice. I guess we just have to hope that if the board's statement was right, the silent majority of the staff are in agreement.