The COPILOT function only has access to data provided through the context arguments. It does not have access to:
Other data from the workbook

You give them too much credit.

There would be no way to do that reliably. There's too much weird stuff people might say to reference things, and the LLM would definitely act on the wrong cells more often than not.

Excel already has a perfectly unambiguous way to provide a specific range of cells, Which is why the =COPILOT() function lets you supply those in the second parameter. I'm assuming they get passed to the LLM as context, likely encoded as a markdown table. LLMs love parsing markdown, apparently.

The user provided no such range of cells, though, so the LLM is most likely seeing none of those other cells, and is just working based on random values from the Internet.