Trust me they can be fully operated without a cloud connection! However if you want to use the tado app then no you can’t without an internet connection. You would also be dependent on their servers.
However if you add them to Home Assistant with the homekit controller you can use them without an internet/cloud connection and you could do all the basic scheduling through Home Assistant (the same is true for using it with Homekit itself).
You don’t need an Apple for this!!! (If you don’t know the difference between the Homekit component and the Homekit controller I will suggest looking this up as the Homekit Controller has nothing to do with Apple!).
I hope this helps, I have been using Tado for over a year now without an internet/cloud connection!