A couple of different things here.
First, smartthings have said for the last year that they are not going to comment anymore on future plans. So I don’t think we’ll see input on that.
Second, HomeKit doesn’t have a cloud. Everything runs locally except for voice recognition through Siri. So all of the device authentication is done locally. Right now the Apple side of the software options discussed in this thread are in early closed beta, but rumors are that it will be released as part of an iOS update later this year. A lot of manufacturers are waiting for that. We’ll just need to see how smartthings responds at that time.
The good news is that if you do want to “roll your own” there is an existing software integration called “homebridge” that quite a few community members are using. It does require having a separate laptop or other device like a raspberry pi to act as a “man in the middle“ server and host the software integration. You can get more information about it in the following threads: