This quote claims it is 'powered by Claude'

“JetBrains and Anthropic share a commitment to transforming how developers work. Developers rely on Claude’s state-of-the-art performance in solving complex, real-world coding tasks. We’re excited to see how Junie, powered by Claude, will help the global developer community create innovative things within the trusted JetBrains IDEs that customers love.” Mike Krieger, Chief Product Officer, Anthropic

Kind of confusingly, in today's release of Rider 'Junie' is mentioned nowhere I can find. The AI assistant tab, which was already available (paid), just has options to pick from popular models (4o, o1, o3, Gemini, Claude) or LM Studio / Ollama