In summary, it's still very early in the adoption curve for home kit and it hasn't been easy or straightforward for companies like Rachio. They shouldn't be criticized at this point for not having this technology.
Just for clarification on Home kit, you need a few items to make home kit work for wifi solutions. Wifi, MCU, and MFI.
MFI or Made For iPod is an authentication chip. Basically is a secure eeprom that is multi sourced from a couple manufacturers and can only be purchased through Arrow Electronics or Avnet. Companies need a MFI license to purchase.
Home Kit software code to run on your MCU.
Wifi: Any will work, Low power low band width work best for applications like this like a 1x1 bgn or AC is supposed to become the future standard. Most wifi solutions have a built in mcu to handle the tcp/ip stack and if there is enough memory you can run the home kit software directly on the wifi mcu. If not, you have to run the home kit software on a secondary mcu like your application processor.
There have been a few solutions that have been announced for home kit support, Qualcomm, Broadcomm, TI, and Marvell, but the devil is in the details of each of these solutions. Some have been recalled due to design changes, some have been delayed, etc. I expect the scenario will change dramatically for the better over the next 6 months.