Hmmm, it is not listed at Rain Sensors FAQ (rachio.com) (only shows the WR2), but a incomplete article is at Rachio compatibility with RAIN BIRD ESP-SMT Weather Sensor
I see it is talked about Rainbird SMT Weather Sensor, Gen 3 controller and Flex Daily - Getting Started / Rachio 3 - Rachio Community and it looks like you hooked up accordingly and it does not seem to be working in that case. Another one: Rain Bird SMT Rain Sensor Compatible? - Accessories & Sensors / Rain Sensors - Rachio Community.
I am not finding a posting that says that it will work, would you mind posting that please?