Imagine Biden becomes the next president. There will be far less outrage porn in a year or two, which would be very bad for Facebook. In other words, less outrage porn = less engagement. Less engagement = less advertisers. Even if the big companies make only 6% or so of their revenue, a 50% drop in engagement will be a tremendous loss for Facebook. Unlike Google ads, which are a function of economic activity, Facebook's ads are not based on search intent. I suppose they could be called more a function of people's psychology and need to engage with social media. So without sufficient levels of engagement, Facebook will lose a lot of money very quickly.
On the other hand, if Trump becomes president again, he will be less likely to significantly curb Twitter, since he needs it really badly. And he is more likely to go after the other social media platforms. At that point he wouldn't even be running for re-election again, so he can act as pettily as he wants and do a lot of showmanship to please his base. Facebook wouldn't want to be on the wrong side of his ledger.
Lastly, there are only a few months to go until the election. The likely damage in that short time span is far smaller than the possible benefits.
Facebook is just hedging its bets here.