No, it wasn't. My wife is a Russian trans girl, and she tried reading the journal – she confirms that it is, in fact, a bastardized fusion of Russian lettering of mostly-English words.
The writer is most definitely not a Russian speaker, and the notebook would be unintelligible to anyone who doesn't speak English.
It's not an attempt at Greek, for sure. She probably used Cyrillic as a simple substitution cipher to stop her parents and others from reading her very violent thoughts.