They were not open about the fact that it was trained on Kimi K2.5. This link explains it better than I can:

https://www.trendingtopics.eu/cursor-admits-composer-2-is-bu...

Cursor seemingly went out of their way to not mention that they were actually running Kimi K2.5 and essentially by that omission made it seem like they had made their own model. They added a note to a blog post about using it at some point and then when they wrote a new one they conveniently left it out again.

That's at least what I perceived as "the drama".