I'm not commenting on the specific case, but just on the nature of what a prenup actually is; I used to think they were mostly used by manipulative people in unhealthy relationships due to money being involved, but my views on them have changed a lot.

So, we have this weird legal institution called marriage. It's basically a specific form of government contract that can only be signed by two people and has all sorts of restrictions around it, but what the contract actually entails depends entirely on where you live since it's a standard contract defined by the government in your area. So not only do you not really get to decide what's in this contract, it's also the ONLY form of contract that provides half of the things it comes with - like automatic hospital visitation rights, tax implications, benefits/coverage rights, estate tax and joint property considerations, etc. If you want some of these benefits to apply to your relationship, there's really no other way to get all of them outside of the marriage contract.

The problem arises because this marriage contract is based on all sorts of social conventions and whatever additional definitions the local jurisdiction comes up with, and as a result it can come with a variety of specific items that don't fit with what a lot of couples want their marriage contract to entail. A prenup is essentially a way of editing the contract so that it fits what you want it to be and works for your relationship, rather than the boilerplate version that the government says it should be by default. It can be about divorce or it can be about any other sort of thing that is important to the couple to define about their specific relationship contract.

Take two people who are in a lifelong committed emotional relationship but who both openly have sex with other people (whether they are polyamorous or just sexually open), and who want to get married because our society provides distinct and unique benefits to married. There are many states where adultery itself is a crime, or where the fact that one partner cheated on the other could be used as evidence in divorce settlements, custody arguments, etc. Obviously no one wants that to happen, but for this couple it may make sense for them to clarify the nature of their physical relationship in a jointly signed prenup to prevent their lifestyle from being a legal liability to them down the line.