It's like using steel to produce steel. What else are you going to use? Bamboo?
The first person to make steel made it without steel didn't they?
Did I miss something?
Edit0: fun tidbit - Wootz steel was made with crucibles of clay with rice husks mixed in (husks would carbonize quickly and introduce air layers to better isolate) and many seemingly random objects (fruits, vegetation) were added to the crucible to control carbon content.
I higly recommend A Collection of Unmitigated Pedantry's series on steel (it's a blog, just search "ACOUP steel".
That's the real problem here - these companies are swimming in money and have armies of humans working around the clock training LLMs, there is no honest reason to nickel-and-dime the actual evaluation of benchmarks. It's like OpenAI using exact text search to identify benchmark contamination for the GPT-4 technical report. I am quite certain they had more sophisticated tools available.