Following the DMCA, a law passed by Congress, will lead to their downfall?

If Twitter gets the DMCA notice and then doesn't take down the video, they're liable for copyright infringement. With the number of views a video like this gets, the damages could be huge. Twitter has no real choice.

There are images in the video from Floyd's memorial service. My guess is that these were taken by someone who is not a Trump supporter. I bet they saw their (unlicensed) photo in the ad and called their lawyer. Their lawyer sent a DMCA letter, and voila.