Yes that is an important advice
In Philips Hue and Ikea Hubs you normally define rooms (groups) and put the lights in these rooms. And then you associate the switches with the groups. In Zigbee world this means that when you turn a room on, one group message is sent.
Maybe you have som scenes with individual settings and then more messages are sent. But overall the number of zigbee messages queued up at the same time is reduced. And remember in a mesh network these messages are relayed by routers so that doubles or triples the traffic.
Home assistant groups are just logical groups and you can mix zigbee, zwave, RF etc devices into same group. But Home Assistant does not know which zigbee groups devices belong to so all it can do is send an individual message to each device.
By defining groups in deconz, these are presented as a single entity in Home Assistant. You can put these deconz groups in a Home Assistant group so you can turn on e.g. 10 zigbee lights in a zigbee group and 2 Tasmota switches in a single action. The result is that one message is sent to Deconz which sends a single turn on message on the network.
There are two things that makes this more robust. I already noted the huge reduction in virtually simultaneous traffic on the Zigbee network. The other is that even if there is a temporary poor mesh route to a single device, the probability that the individual device hears the group messages is high. I have occationally seen a light bulb that did not respond to on off but it worked when I turn the entire room on and off.
And in normal life 90 % of what we do is turning rooms or groups on and off
And finally for dimming, use this method… Never step increase or decrease a large number of zigbee devices.