All the other compilers in the comparison are C compilers, right? Whereas this compiler is compiling its own home made language? So not sure how the comparison can be valid.
I agree with you though on a different note. It's dubious to compare compilers by benchmarking them, because tests are highly arbitrary and are won/lost based on single weak links. It's not really an exact science, but rather something you can start with to figure out how things are behaving. I mostly base my opinions by looking at the assembly code each compiler generates, but a single bar graph is a better presentation for articles.
It has been released as open-source together with the binaries, https://atariwiki.org/wiki/Wiki.jsp?page=Action
You should be able to run these using an Atari 8-bit (XL/XE model) emulator, like Atari800, https://github.com/atari800/atari800/releases or Altirra, https://www.virtualdub.org/altirra.html
GNU Compiler Collection no longer targets 6502, but an older version of the GNU C Compiler could. LLVM targeting 6502 is limited. There is also the cc65 cross compiler. [1]
LLVM [2] and GCC [3] are compiler and toolchain technologies. The name Low Level Virtual Machine (LLVM) is no longer officially an acronym, and the GNU C Compiler is now the GNU Compiler Collection (GCC).
[1] https://www.cc65.org/ [2] https://www.llvm.org/ [3] https://gcc.gnu.org/
Likewise, GCC contains a C compiler but the machine specific parts like the codegen take an intermediate representation (GCC has several) as input.
Almost no compilers out there compile directly from C to machine code, there is one or more intermediate forms in between. C is not a good input language for an optimizing compiler.
It's a good and entirely valid comparison. It's the backend codegen that is being compared, the language frontend does not really play a part in it.