Reading that paragraph about licensing it came across as partly concern about GPL and partly about other stuff. GPL3 is more restrictive, and if you're dependent on a library which changes license from say 2->3 you'd have the choice of maintaining your own fork without patches or moving to the new license. Not sure why that would be an issue for them specifically but it's certainly possible it would impact their business, and is a slight concern, even if unlikely.

Often this sort of decision is based on a mix of concerns, and it also sounds like they wanted this core feature to be easier to iterate on quickly and be without dependencies, which I can understand. Quite possibly if the rest of their code is in Go it just felt a nice fit to have this important piece also in Go instead of contributing to a C library which has a different focus.