I'm surprised the proposed initial scope would include x86-64 but not ARMv7 or AArch64. That suggests Google's motivation here is their server software, not mobile devices running Android or Fuchsia.
Also, portability and consistency is an important attribute of a libc. Supporting multiple architectures from the beginning would help shape a portable design. For example, the original target processor for Windows NT development was the Intel i860 specially because it wasn't the standard i386 architecture. Fun fact: the Intel i860 was codenamed "N10" aka "N-Ten", which may have been the origin of the Windows "NT" moniker.
Also, portability and consistency is an important attribute of a libc. Supporting multiple architectures from the beginning would help shape a portable design. For example, the original target processor for Windows NT development was the Intel i860 specially because it wasn't the standard i386 architecture. Fun fact: the Intel i860 was codenamed "N10" aka "N-Ten", which may have been the origin of the Windows "NT" moniker.