It would require the aqara hub, as I mentioned.
But it shouldn’t mean cloud to cloud integration if smartthings does its side of matter support correctly, which it looks like it does, because it’s already partially coded into edge.
Matter is intended to be “local first,” with a high priority on local communication and processing.
Aqara hubs already operate locally in HomeKit setups, so I feel confident that they will have this done. But it does depend on what smartthings does on its side.
Just as an example, right now the same Meross WiFi smart plug will show up simultaneously in both smartthings and HomeKit.
The HomeKit integration is local, and will continue to work fine if the Internet goes out as long as the local Wi-Fi is still working. (The HomeKit app does not require Internet.)
The smartthings integration is cloud to cloud, so if the Internet goes out or the smartthings cloud is unavailable or the Meross cloud is unavailable, you don’t have control. But then, if the Internet goes out or the SmartThings cloud is unavailable, the smartthings app doesn’t work either. 
So…yes, local operation would require a $60 aqara hub to get Matter integration. But as long as smartthings does its side in the preferred manner, The integration will be local, not cloud based. 
I’m not saying that will be the solution for everyone. But there will likely be a significant number of smartthings customers who will be happy to use the inexpensive good quality aqara devices in SmartThings via Matter and an aqara hub. Spread the $60 across 10 aqara sensors and the total cost will still be a lot less than many competing brands. Choice is good. 