Diane makes everything about herself. She is totally self-absorbed, and never really gives consideration to anyone else’s feelings or experiences. That’s established from the very beginning, when she’s hired to ghost write someone’s autobiography, but instead writes a book all about her experiences with that person and puts her own name on it. And then leaks part of the book to spite him for calling her out on not doing what she was hired to do.

So of course, when she hears about what Bojack did to Penny her reaction was not “how could you do that to her?!” it was “how could you do that to me?!” And she proceeds to yet again use the access provided by her position as a writer for spiteful purposes. What happened to Penny meant nothing to Diane beyond how she wound up looking like a hypocrite for supporting someone who would do what Bojack did, after all her self-righteous and judgmental posturing. That’s why she felt “betrayed” by him.

It never even occurred to her, when she sat down to write that scene to lash out at Bojack, that she was exploiting a vulnerable young woman’s trauma for spite. That is a truly repugnant and disturbing display of main-character syndrome. And she hasn’t changed much by the end of the series.