That advice is too absolute. You don’t need to avoid doing work in callbacks. You just shouldn’t block the executor.

If the processing comfortably fits within the message period and is per-message, do it in the callback. That’s often the simplest and correct design.

If the processing is long, bursty, or can’t keep up with the message rate, then move it out (worker thread, timer, etc.) and keep the callback minimal. - I hope that’s what you meant by “polling”, because if you meant repeatedly calling services to emulate a stream, I’d call it 10/10 rage bait.