> But I keep wondering if they could integrate at a lower-level than the source code.

I’m sure they could, but targeting go source code has the benefit of giving early adopters an escape hatch. If it targeted LLVM directly, I would never consider using this at work since the risk of it being abandoned is too high. But since it targets go source, I would perhaps consider it for some low importance projects at work.

The standard go toolchain doesn't use LLVM. Go has its own assembly format and machine code generation.