Error with `adb sideload`when trying to install Android 12 in recovery mode

According to Google's "Get Android 12" guide,

The easiest way to get Android 12 Beta on a Google Pixel device is to enroll your device in the Android Beta for Pixel program.

but also

Alternatively, you can flash or manually install the latest Android 12 Beta build on your Pixel device. These approaches can be useful when you need more control over testing, such as for automated testing or regression testing.

I had previously joined the beta program to test Android 11 developer preview so for Android 12, I thought I'd try the flashy way.

I followed this guide to flash the update to my Pixel 3XL. However, when I finally typed the adb sideload command and pressed "Enter", it gave

adb sideload ~/Downloads/crosshatch-spb4.210715.011-factory-cbdbee68.zip 

adb: sideload connection failed: no devices/emulators found

adb: trying pre-KitKat sideload method...

adb: pre-KitKat sideload connection failed: no devices/emulators found

and adb devices doesn't find any devices connected, although my Pixel 3XL is indeed connected through the cable, and is in recovery mode, up to step 6 of the guide. How do I get pass the error?