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I think you are overestimating the impact of the long tail of compiler optimizations. You don't need to reverse-engineer every microarchitecture under the sun and optimize for their specific quirks like LLVM does to have useful code generation.

Just doing the basics goes a long way with a tiny fraction of the effort. Yes it will leave some percent on the table but this is hardly the end of the world. With their own compiler they have full control over the entire chain and might be able to make up for that with their own language-specific optimizations.

Go has a custom compiler, too, it's not as good as LLVM, so what?