arm64 is not listed as a supported arch at https://packages.gentoo.org/arches
For example https://packages.gentoo.org/packages/sys-devel/gcc does not list arm64.
arm64 is experimental and needs some TLC but Gentoo could advertise arm64 support.
It's not listed because it's not a fully "supported" arch. This has long been the case for several other arches that are experimental, that may not even have handbooks or much other documentation. Examples: MIPS, Prefix, etc. Until and unless the arm64 maintainers indicate their arch is as supported (~ or stable) as any mainstream architecture, it won't show up. CCing the arm64 folks just so they're aware, if they have any input.