They curated feeds specifically trying to find offensive content and then refresh their feeds until they could find an ad placed near offensive content.
You could pull this trick on any tech company: Google, Facebook, etc. But Media Matters hates Twitter/X and Musk, so they're focused there.
People forget that Media Matters is a political organization, created by a Clinton loyalist, that works closely with a number of super PACs also created by David Brock. They target their political enemies and that's it.
Anyway... these advertisers will be back. This weekend's fiasco about OpenAI proved that there is a need for X -- none of the breaking news was coming from Threads or some random Mastodon server.