Bot API 429 errors during load test spike

Context:
Running JMeter scripts against /api/v2/engagements/bots to simulate 500 concurrent starts. Using SDK v2.10. Rate limits seem to kick in instantly.

Question:
Could someone explain why the platform returns 429 Too Many Requests when concurrent connections exceed 50? The docs mention burst capacity, but my tests fail hard. Is there a specific header to tune for higher throughput in Architect flows?