Why does Brühl conclude that silent complicity does not create a moral obligation for managers of companies in Russia to speak out?

Because companies are not obliged to speak out when the state threatens retaliation they cannot realistically fend off, and Russia criminalises even calling the war a "war", with up to 15 years in prison.

The argument for silent complicity is that multinationals are powerful, so they could act against human-rights violations and are complicit if they stay silent. The limit: a moral duty to speak up does not hold when doing so would expose the company and its people to state retaliation they cannot resist. Russia punishes the use of the word "war" with up to 15 years' imprisonment.

So resisting human-rights violations connected with the war is not a moral obligation for company leaders in Russia. Silent complicity does not, on its own, justify the demand that they leave.