To my mind, as a network engineer, what smartthings does is very annoying. But they’ve always done it the same way.
They do follow the Z wave standard (that’s how they get their own devices certified), but then to use the smartthings app You have to also follow their own proprietary architecture layer, which they put on top of the independent third-party standard for both Z wave and Zigbee. And that’s where things start to break down.
Device manufacturers who already spent time and money meeting the independent third-party standard usually don’t want to invest in something that’s specific only to Smartthings. A few of the smaller companies like Zooz and Inovelli do make that extra effort, but most of the bigger companies don’t.
So you end up with a device which is certified for Z wave and a smartthings hub which is certified for Z wave and you still can’t add your device. Makes me crazy. 
If the device itself is popular, there will usually be a community developer who will create a custom edge Driver for it, so that it can work with the smartthings platform. But then not only are you, depending on the community, smartthings support usually won’t help you if you run into a problem because you’re using custom code.
It sounds like you’ve got the problem solved for this particular device since the manufacture created their own custom edge Driver.
In case you run into this within future Z wave devices, The community is very helpful and very creative and you can request that a community developer create a custom edge Driver, and that may or may not happen. But the official channels can be very frustrating.
If you’re interested in asking the community developers, you can do so in the following thread. Read the first post carefully so you know what information you have to include in your request so that the developers can help you.
Post Requests for Zwave Edge Drivers Here (community-created)