I wonder if those same senior employees signed the letter. If the WaPo could verify that, that could clarify whether peer pressure extended beyond Ilya.
It wouldn't be surprising if they did sign the letter, and it wouldn't take peer pressure to make it happen. People can have concerns about a manager's style or a leadership culture and still not want to have years of lucrative equity comp erased overnight by the board's reaction to those concerns.
I'd still find the added context insightful. They went to the board alleging psychological abuse, so I assume they understood the board would at least consider removing Sam.