Is that still true though? It was definitely the case some 5-10 years ago, but last time I checked they were pretty comparable, losing some, winning others.
If I were writing a program that needed to be really fast I would just compile with both on max optimizations and pick whichever was best. Most software isn't the linux kernel and will work fine with both compilers
For instance, GCC is by far the dominant compiler on GNU/Linux on x86, while LLVM has a almost total monopoly of AArch64 (think about Android NDK + iOS + macOS). I think it would be quite natural for the GCC AArch64 backend to get less focus compared to its x86 counterpart. It's a complicated landscape that is always full of gotchas, and that's why I don't like these kinds of benchmarks a lot.
The key takeaway is that GCC is still an extremely good compiler.