This post seems to lack any useful information to come to the conclusion of "phoning home the content of PowerPoint slides". That said, if you use the tool in question, you'll notice that it only suggests layouts and these seem to be fairly generic. They don't seem to take into account the content so much as how the content is laid out.
This could be as "simple" as Microsoft phoning home with the layout of the data (bullets vs title vs paragraph of text) combined with perhaps hash codes of any topical features (things that indicate technology vs food).
I've never been "surprised" by the feature the way I am with Github Copilot where it sometimes feels like it's reading my mind. The Designer is just a simple way to click a button and get back 5+ recommendations on how you can layout the data.
Without some kind of evidence, my explanation is just as valid as the blog. That which can be asserted without evidence can be dismissed without evidence.