It works:
https://youtu.be/qpTq6zRP3yw
How to connect:
1) Update your devices to the latest iOS (14.4 in my case).
2) Check the hotspot on the iPhone and rename it in case it's too long e.g. "iCar"
3) Remember the hotspot password
4) Reset your HomePod to factory settings
5) Create a new wi-fi network at your home router with the name of your hotspot e.g. "iCar" and the same password
6) Connect your iPhone to the new network and setup the HomePod
7) Turn on the hotspot on the iPhone and with the wi-fi network on the router off
8) When you're in the car, use the 20 Watt USB-C adapter, switch the hotspot on and wait for connection.
It may take time for the HomePod to connect to the hotspot. Make sure the Home app is up and running on the phone, also switch off all other apps.
I tried switching the hotspot's setting "Family Sharing" on and off to push the connection, it helps randomly.
The time to connect takes from few seconds to several mins, and I can't understand what it depends on.