It it because of the much larger security overhead of Zigbee 3.0 compared to older Zigbee protocols such as Zigbee Home Automation 1.2 and Zigbee Light Link 1.0 which does not have overhead when adding more devices. So you have to understand the different RAM footprint overhead between Zigbee 3.0 devices and older devices.
The main bottleneck is the RAM memory on the SoC and Zigbee 3.0 devices connected to a native Zigbee 3.0 Zigbee Coordinator will use higher security level (encryption, etc.) which takes up a larger footprint in RAM per device.
So the fact is that if you only connect newer Zigbee 3.0 devices then the Sonoff ZBDongle-E and SkyConnect baded on EFR32MG21 will run out RAM after connecting abond 200 Zigbee 3.0 devices, but if you instead use older Zigbee devices (i.e. Zigbee Home Automation 1.2 and Zigbee Light Link 1.0 devices) then you can connect several thousands of such devices as they do not use much RAM, and obvoisly a mix of newer and older devices will result in a number in between.
Future SoCs such as the EFR32MG24 feature more RAM and faster CPU so they will be able to handle more Zigbee 3.0 devices than EFR32MG21 that have less RAM.
Alternativly if you are willing to sacrifice security then you can get around this by connect your Zigbee 3.0 devices to a older Zigbee Home Automation 1.2 Coordinator (like example ConBee 2) as then all devices will connect in backwards compatible mode and not use the Zigbee 3.0 security.
So yes, you can connect thousands of Zigbee devices, but not if they are native Zigbee 3.0 devices connected to a native Zigbee 3.0 Coordinator like EFR32MG21 based adapters are.
That is a theoetical limitation if you had unlimited RAM and CPU resources on the Zigbee Coordinator SoC, while in practice with newer Zigbee 3.0 security overhead you will run out of RAM and CPU (especially RAM) on even the latest Zigbee Coordinator SoC.
To futher clearify, you will run out of RAM on the SoC long before you reach that limit and after that you run out of CPU on the SoC. Thus the RAM and CPU on the SoC is the bottleneck if want to connect more Zigbee 3.0 devices, which is why we can look forward to newer Zigbee Coordinator adapters based on example the new EFR32MG24.
Yeah, those are not Zigbee 3.0 devices connected to a Zigbee 3.0 Coordinator. Zigbee 3.0 devices only started to become available on the marked in 2016 by early adopter manufactuers and most other companies did not start to release Zigbee 3.0 devices until around 2019. So chance is that if you have a Zigbee device that was released more than 5-years ago then it is not a Zigbee 3.0 devices.
Alao note that the popular ConBee 2 and RaspBee 2 are not Zigbee 3.0 Coordinators so all devices will connect to them in backwards compatible mode and that is why you can connect more Zigbee 3.0 devices to them than you would expect to a older Zigbee Coordinator, but again you sacrifice security when you do so. (It is like connecting an newer WiFi device to an old WiFi router / access point where they can only connect to the newest protocol version that share).