Pretty much every ZigBee hub will lock out third party devices, because due to how ZigBee works, either you need to provide a raw API to endpoints (which can be dangerous for the average person), or carry the device definitions yourself (some of those are covered by the standard Zigbee endpoint assignments per spec, but not all manufacturers follow them, many extend them - looking at you, Philips - or completely ignore them).
Allowing third party devices is also a security and stability risk. A good example would be the handful of cheap Tuya presence sensors based on cheap 5/16/24GHz mmWave radar tech. They're really inexpensive, small, and work over ZigBee. Awesome, right? Well, no. They literally spam the network with unnecessary updates. A single one of those was responsible for ~30% of all messages on my ZigBee mesh of over 30 devices. It literally sent as many messages as ~14 other devices (many of them relays, this Tuya sensor is endpoint).
Now imagine if Aqara allowed their hubs to connect with any ZigBee device. Imagine if someone bought a dozen of these presence sensors and installed them... And the ZigBee network immediately became so overloaded that a simple light switch event takes minutes to trigger. People would be upset with Aqara, not the manufacturer of the sensor.
Simply said it's easier to exclude third parties for sake of user experience that can be guaranteed.
However that doesn't mean Aqara isn't shitty about ZigBee. Most of their non-hub devices for example simply refuse to join other ZigBee networks because they use a proprietary pairing method that first talks to the device, phone, and hub via Bluetooth, exchange pairing keys and only then will it allow pairing between device and hub. And of course the phone app won't let you add a third party hub to do this with, so any Aqara ZigBee device that uses this approach (e.g. their T2 bulbs) won't work with third party Zigbee hubs.
(Fortunately the T2 is a dual mode bulb that can use Matter over Thread which I opted for in my installs)