I covered the reason for that here → https://community.home-assistant.io/t/guide-for-zigbee-interference-avoidance-and-network-range-coverage-optimization/515752
To summerize it is easier to just recommend a powered USB 2.0 hub over a unpowered USB 2.0 hub because many users commonly use a single-board computers with a power-supply that is just barley has enough power to power the single-board computer itself, without any pheriphials, then they normally connect an unpowered harddrive + radio dongles (all of which adds to the total amount of power needed) and then it very common to get get intermittent issues which the root cause turn put to be power related problems.
Simply put, if you use a powered USB 2.0 hub then you have at least secured powered to the radio dongle so any issues you might still get should not have to due with that radio dongle not getting enough power.