the pi pico is 100% supported in Zephyr. https://github.com/zephyrproject-rtos/zephyr/tree/main/board... Did the author not check the docs? https://docs.zephyrproject.org/latest/boards/raspberrypi/rpi...

Additionally, you aren't intended (for many situations) to use a single "main" Zephyr install, but to include what external modules you need in your project's west.yml. If you have a number of projects sharing the same Zephyr install that's a separate discussion but installing every possible toolchain/HAL is not the only way to do things.