Maybe it's just Microsoft moving to more model agnostic tech within their copilot. I recently started using Microsoft 365 Copilot because corporate added Cowork which runs on Opus 4.7 which was better than the alternative we have available. Unlike the "real" Claude Code or Cowork this only has access to files in a specific onedrive folder in your personal sharepoint container, so it's much more compliant to things like NIS2.

Technically we're using Copilot and we're playing for it through Microsoft licenses, but it's using Opus 4.7. Even before this, most of our custom agents within m365 copilot were one of the GPT models.

Or maybe you're right and they want their developers to use the copilot models.