> What if we don’t have control over the frequency of requests (e.g. using a service like Feedly)?

I believe most RSS reader-clients and aggregator backends are programmed to respect HTTP cache-control headers; so as long as the developer of this service sets those headers appropriately for their endpoints, there shouldn't be a problem.

The warning is likely more for people's custom scripting using curl(1) et al, where there isn't an HTTP cache in the code path.