Admittedly I missed that assumption in cutting to the meat of the article. However, does WordPad or a spreadsheet really need to synthesize styling when the font doesn't have it?
I just loaded a font without an italic style into TextEdit on macOS, and it won't let me italicize it. It doesn't synthesize italics. I'm assuming the same goes for the rest of Apple's apps.
The article specified an assumption that this is about laying out editable rich text, where the user using the program is able to arbitrarily style the text as part of using the program.
Like writing HTML+CSS for a web browser, yes; but also like WordPad, or like a cell in a spreadsheet program, or like the contents of a label field in a view builder. And also like changing text display preferences in a terminal emulator.