The documentation at https://source.android.com/setup/build/initializing actually does say something about the host architecture. The package names imply an x86_64 host that also supports i386 binaries via multiarch.
Some users have attempted to run the AOSP build flow natively on aarch64 systems, but this is unfinished research.
A means to build AOSP on a Pi requiring not as much creativity would be to install
qemu-user-static and develop inside a Debian x86_64 chroot. This can be done even from a 32-bit distribution such as Raspbian. Keep in mind, although you have said:
I can live will long build times
the limited horsepower of the Pi 4 combined with an order of magnitude slowdown due to emulation may result in a build time of days to weeks.
The Pi 4 is marketed as a desktop replacement, not a development workstation replacement.
 C/C++ Toolchain for Android Build on aarch64 (WIP)