The article mentioned that the use of 'ASCII' within the context of those tools should not be seen as the limited character set ASCII. Personally, I would avoid mentioning ASCII at all.

The title just talks of plain text though, and plain text usually means UTF-8 encoded text these days. Plain, as in conventional, standardised, portable, and editable with any text editor. I would be surprised if someone talked about plain text as being limited to just ASCII.