Because the string "Buildroot" isn't known to Virtualbox, when creating the machine it offers type "Other" and version "Other / Unknown".
As the name seemed very generic, I presumed this meant that it was agnostic bitwise*.
Under the bonnet, this is a 32-bit option and there is a "version" called "Other / Unknown (64-bit)".
As soon as I had changed this ...
The pc_x86_64_bios_defconfig seems to be 64bit. In Oracle VirtualBox, hardware support for virtualization (Intel VT-x or AMD-v) is required for all 64bit VMs, even when the host OS is 64bit. KVM just does the virtualization differently.