It’s not absurd at all. They know the IPs, they know those devices use the same network, and they also know where they are located pretty accurately: the Google Street View cars also scan for WiFi networks and map them to their location.

2 devices consistently connected to the same router, to the same network, in the same place… must belong to the same person or to 2 people sharing a home. If cookies set by other websites and seen by Google show similar browsing habits, it’s probably the same person.

You just mentioned 2 different Google accounts: if your devices are connected to Google accounts they are already getting a lot of information from you that way, and Google knows that those 2 accounts are related.

I use a phone holder and my phone sits there with my maps app showing me the navigation.