Aider author here.
Based on some DMs with the Gemini team, they weren't aware that aider supports a "diff-fenced" edit format. And that it is specifically tuned to work well with Gemini models. So they didn't think to try it when they ran the aider benchmarks internally.
Beyond that, I spend significant energy tuning aider to work well with top models. That is in fact the entire reason for aider's benchmark suite: to quantitatively measure and improve how well aider works with LLMs.
Aider makes various adjustments to how it prompts and interacts with most every top model, to provide the very best possible AI coding results.