We all have mean thoughts. We (at least most of us) learn to keep them to ourselves. Maybe writing it out helps her to cope with the fact they aren't "young hot college athletes" anymore. Maybe she used to be a bully that made fun of other kids for her own insecurities, has grown and realized it's exactly how not to have good relationships, so she took to writing mean things to feel better about herself, and he just happened to find one about himself.

OP has given no indication that she has actually acted like this is the way she really feels.

I tend not to spend my time speculating what she may or may not have meant. I am purely using what was given. Also she really does feel those things when she is writing them, even though they might those feeling might just be in the moment.

not indicative of true feelings

Again maybe we will have disagreement of what is meant here by true feelings. I personally believe they are true feelings just not filtered like they usually are when we interact with others.

I have ADHD so I am used to overthinking and processing thoughts similar to a journal but it's all in my head. However, I don't believe there is an excuse for what you say or write just because you don't intend someone else to see them. Nothing I think or write would be something I would not say to my partner or have a discussion about despite it being uncomfortable. If I was uncomfortable with their weight I would say so, not write in a journal I think they are becoming a fat slobbering pig and they disgust me.