Just to be sure: the post does not say that the performance of Open LLMs is plateauing (because that would be false, e.g. Google just released a Gemma2 that blows out of the water all previous open models from the same sizes).

Its true title is "Performances are plateauing, let's make the leaderboard steep again", which means "on the Open LLM leaderboard, top models have basically reached a point where they've all grokked the benchmarks which makes it harder to distinguish them, so let's change the benchmarks for harder ones to make a difference again."