It’s true that if the ST hub were HomeKit certified it would get some control over Lutron Caseta devices. Not “the same way it would any other device that is directly paired to the hub”–see the Insteon HomeKit-certified hub for a good example of some of the limitations.
All of that said, although back in January 2015 there were some discussions that ST might go for HomeKit certification, they dialed back on that by March 2015, as did many other device manufacturers, once more information about HomeKit was available.
For a detailed discussion of HomeKit and ST, see:
If you look for additional references, check the date and stick to things published in March 2015 or later. A lot of the early assumptions about how HomeKit integration was going to work turned out to be incorrect.