sys-apps/thunderbolt-software-user-space-0.9.3 does pkg_pretend() { CONFIG_CHECK="THUNDERBOLT" ERROR_THUNDERBOLT="This program talks to the thunderbolt kernel driver, so please enable it." CONFIG_CHECK="HOTPLUG_PCI" ERROR_HOTPLUG_PCI="Thunderpolt needs pci hotplug support, so please enable it." This prevents a binary package being built on one system, that may not have the options set, for use on others where the kernel is correct. Custom and practice is that the ebuild should warn about required unset kernel options but not prevent the build from completing. With the quoted section commented out it builds on an arm64 box where I have no access to fix the kernel.
Strictly concerning ~arm* keywording, I'm not sure it even makes sense: I've masked these flags on arm: ># Andreas Sturmlechner (25 Feb 2019) ># (SM)BIOS/Thunderbolt outside of amd64/x86? - citation needed >sys-apps/fwupd dell thunderbolt
Thunderbolt support for arm64 appears to have been in the learn in the early days of thunderbolt buts its gone now so sys-apps/thunderbolt-software-user-space cannot be useful on arm64
