I think there is a sense that if Apple is willing to walk away, it’s a signal to other companies that the problem is real.
I agree that Apple may be the canary in the coal mine but I propose a motive beyond ethics.
It may be that Apple has seen a reduction in traffic from ads on Twitter recently and this latest move by Musk, coupled with everything else, gave them leverage to get out of any contracted ad buys. They pull ineffective ads, help their brand and ethics image, and save money doing it.
The cynic in me says whatever the motivation the largest driver is financial.