For the most part yeah. There are several soundtrack composers that are notorious for stealing - and a lot of soundtrack composers listen to other peoples work to get ideas, inspiration or downright copy it. So basically there’s two issues, artist value (ie. is someone really unique or great to be copying another’s work?) and there’s ethics and legality. Copyright laws pass around 75 years (if I remember right) after someone’s death, which is why classical music is often a safe zone for others to copy or get inspiration from - they are the masters of music that composers study and strive for.
There’s a huge amount of data you can find on the internet, YT, etc comparing her works side by side with the artists she steals from. It is staggering, everyday Kanno fans are finding more and more.
As far as Horner, yeah he’s one of the most infamous ones around. You might know him from the music by Clannad… er I mean to Titanic. His score to Honey I Shrunk the Kids had a suit against it and Disney had to settle out of court - since then I don’t think Disney has hired him again and he’s been a joke in some of the more academic circles.
Soundtrack composers like Kanno, and other big name ones usually have a production staff and ghost writers that help them compose the music as well. It’s kind of a shady scene, unfortunately. One never really knows the true extent of how creative or talented some musicians really are - most people don’t really know what goes on behind the scenes, since we can not see it tangibly.
Music is a very tricky art, and as far as I’m concerned one of the most complex sciences, because it’s not as easy to detect plagiarism. It’s not like seeing a copied word for word sentence or seeing someone draw and color the same thing. Music has a lot of intricacies and most people simply don’t have the ears to detect - as it’s only acquired either through dedicated study or natural born gift. Perfect pitch and even relative pitch isn’t common with the general population at large. So most people don’t know any better, they just have the basic feel of a song or whether or not something sounds nice to them.
To be clear, I’m not saying she is not talented. She makes nice sounding music, however I find her uniqueness and ability to express her own style questionable. And in my book what she does isn’t genius. For all I know the true genius is someone in her production staff or ghost writer.