The function signature does allow that possibility, but is documented as "undefined behavior" in a sense - as the purpose of this callback is to have access to the RedisClient for invoking commands.
RedisPipeline implements RedisClient to handle the enqueuing of commands through send(command:with:).
I wrote an issue to at least improve the ergonomics of execute(): nio-redis#28
I have a feeling that while working on that, the enqueue(operation:) signature will be more restrictive to prevent inadvertent API misuse