"Agenticness" depends so much on the specific tooling (harness) and system prompts. It mentions Copilot - did it use this for both? Given it's created by Microsoft there's good reason to believe it'd be built yo do especially well with GPT (they'll have had 5 available in preview for months by now). Or it could be the opposite and be tuned towards Sonnet. At the very minimum you'd need to try a few different harnesses, preferably ones not closely related to either OpenAI/MS or Anthropic.
This article even mentions things like "Sonnet is much faster" which is very dependent on the specific load at the time of usage. Today everyone is testing GPT-5 so it's slow and Sonnet is much faster.