This event was more than just a costly signal. The costly signal would have been "stop doing what you're doing or we'll remove you as ceo" and then not doing that.
But they did move forward with their threat and removed Sam as CEO with great reputational harm to the company. And now the board has been changed, with one less ally to Sam (Brockman no longer chairing the board). The move may not have ended up with the expected results, but this was much more than just a costly signal.
As mentioned earlier, methane breaks down after 10 years into CO2 which makes livestock more part of the cycle rather than a producer. Applying the tax to any process that directly removes carbon sinks (eg. drilling/forest clearing) should take care of any downstream processes that rely on it.