You're reading the documentation for a version of ROS 2 that has reached its EOL (end-of-life), and is no longer officially supported. If you want up-to-date information, please have a look at Humble.
ROS 2 on Raspberry Pi
ROS 2 is supported on both 32 bit (arm32) and 64 bit (arm64) ARM processors. However, you can see here that arm64 receives Tier 1 support, while arm32 is Tier 3. Tier 1 support means distribution specific packages and binary archives are available, while Tier 3 requires the user to compile ROS 2 from source.
The fastest and simplest way to use ROS 2 is to use a Tier 1 supported configuration.
This would mean either installing 64 bit Ubuntu on to the Raspberry Pi, or using the 64 bit version of Raspberry Pi OS and running ROS 2 in Docker.
Ubuntu Linux on Raspberry Pi with binary ROS 2 install
Ubuntu for Raspberry Pi is available here.
Make sure to confirm that you have selected the correct version as described in REP-2000.
You can now install ROS 2 using the normal binary installation instructions for Ubuntu Linux.
Raspberry Pi OS with ROS 2 in docker
Raspberry Pi OS 64 bit version is available here.
Raspberry Pi OS is based on Debian which receives Tier 3 support, but it can run Ubuntu docker containers for Tier 1 support.
After flashing the OS, install Docker.
The official OSRF ROS 2 Docker container definitions can be found here.
You may choose from ros-core, ros-base, or ros-desktop. See here for more information on these variants.
Clone the docker_images git repo onto the Raspberry Pi, change in to the directory linked above, then to the directory with your preferred variant.
Inside of the directory, build the container with:
docker build -t ros_docker .
On a supported system it will only take a minute or two to build the docker containers, as the source code is already built in to binaries.