No, I don’t think it’s that MCs are doing a good/better job.
The 20% is based on the affair only, meaning there were no other “major” problems in the marriage.
Marriage is a pretty strong bond between people. They may stray but that doesn’t mean they are trying to get out of or don’t want to remain married.

Many of these people do successfully recover on their own. Some simply gloss over it and never mention it again, some put up with it such as having a mistress as long as it isn’t discussed and others stay together and are miserable.

That perhaps infidelity isn't the horrible kiss of death we've all been taught?
Exactly! It’s extremely horrible and painful but it far from the kiss of death.

But, if an affair is the very worst thing one spouse can do to another, why is that number so low?
Why would only 20% of the marriages end?
Again, this percentage is based on an affair only, not other problems in the marriage. Usually, if there is infidelity, there is other major stuff going on too.

Here is another interesting fact.
20% of people who file for divorce, never complete it or withdraw it. They may reconcile, never see each other again or whatever, but they don’t end the divorce one way or another.

Could the number of divorces vs. the % of infidelity be so skewed because the BS often doesn't know?

I think it could change the numbers a bit but probably not that much (1-2 points?)