I ran into the same issue yesterday and stumbled across a fix that worked for me.
- I attempted to set the connection with HomeKit through the Rachio App wizard and it kept responding with an error about unable to connect to device.
- I was attempting to also set up a new garage door controller and had to do that manually.
a. When I was adding the garage controller manually the Rachio Controller showed as available to add through the HomeKit App so I tried adding from Homekit interface instead of the Rachio App. When I did this it prompted for a 6 digit code which I did not have.
b. I went back into the Rachio App to see if I could find a 6 digit code and when I launched the Rachio HomeKit wizard it immediately gave me the 6 digit code.
c. I went back into HomeKit and manually added the Rachio controller using the 6 digit code and it added fine. - It now works fine using Siri to start and stop Zones.