But then they go on to say that their "previous binutils used" was https://github.com/myri/lanai-binutils.

It certainly seems believable that this is a third-party processor architecture, but Google has a contract with them to build specific models of that processor that meet their needs (possibly alongside some other proprietary hardware, like a high-speed NIC), and those models aren't sold to the general public. That's pretty common outside x86, right? For instance, is there a way for me to buy a BCM2835 other than by buying a Raspberry Pi?