The Valkey implementation of multi-threading is fundamentally different than what existed in Redis. The history dates back to work done in ElastiCache that was released as "Enhanced IO", https://aws.amazon.com/about-aws/whats-new/2019/03/amazon-el.... The version released in Redis could only do about 350k RPS because of poor memory locality of operations, the inability to do command processing while handling I/O, and the inability to offload much of the TCP read path. The new version in Valkey can achieve 1.2M RPS.
"They made certain improvements later", should be "we threw away the old implementation and built a better one."
"They made certain improvements later", should be "we threw away the old implementation and built a better one."