There is no reason they couldn't have done this without DoH being an available standard. Remember, DoH is just an ordinary HTTPS request. Anyone can hide almost anything in that, including their own homemade DNS.

Your concern about smart devices using DoH can be trivially avoided by not buying smart devices that use DoH, or even connect to the internet. Make sure that whatever you buy works with a local server such as Home Assistant and is actually under your own control. A system like this will be more reliable and customizable too.