> but it doesn't operate within Excel's data model

Power Query is an Excel add-on, and has been around much longer than Power BI and has nothing to do with it. This has been possible for almost a decade, since Excel integrated a robust in-memory columnar database capable of dealing with millions of rows.

> it would be wrong to pretend like the join is done in Excel

The join is done in Excel, I don't need to pretend. That you think Excel is just a bunch of sheets and cells is your own mistaken mental model of what the program is and can do.

Here's another reference:

"Now that Excel has a built-in Data Model"

https://support.microsoft.com/en-us/office/create-a-relation...