So good (and bad?) news 
I had bought a new dimmer remote a few weeks back to go with my 2+ year old ones and despite the Zigbee signature details being the same, showing the same firmware, showing the same quirks… this “new” device has your zha_event structure whereas my “old” three devices do not. The model number on the back is the exact same, too.
What I can’t figure out is I would generally assume the stock would have been rotated out over this many years such that a new device is more likely to have firmware updated past the last time I had a Hub (probably close to three years). But in the other thread that person said they updated their remote and now see the less featureful zha_event.
Not sure where that leaves us all told except, there’s clearly a firmware-dependent difference (which is good we eliminated Zigbee radio/HA stuff? I am not clear what firmware that actually is, except to guess it’s very old.