>> Who would want to use a distributed key-value cache as a rate limiter?

> Who would want to? APISIX apparently

There is a difference between using a technology for which it is intended (APISIX) and what it may use as an implementation detail (Redis).

For example, many services use a persistent store such as MySQL or PostgreSQL in their implementation. This does not imply either are sufficient to completely replace the functionality provided by said services.