Just doing some testing the other day but not sure of the current status of button being exported to home kit is... I set up a virtual switch and exposed it to HomeKit integration. Came through fine. As another test I changed the driver from Virtual Switch to Virtual Button and HomeKit comes back as unsupported.
As you can see in the first pic the test switch with the virtual switch driver is fine. In the second pic it's using the virtual button driver and says not supported. Tagging @gopher.ny