I can't tell what the argument is just from the slideshow. The main point appears to be that code pages, UTF-16, etc are all "plain text" but not really.

If that really was the argument, then it is, in 2026, obsolete; utf-8 is everywhere.

He has a YouTube channel, there's a talk on there.

He also discusses code pages etc.

I don't think the thesis is wrong. Eg when I think plain text I think ASCII, so we're already disagreeing about what 'plain text' is. His point isn't that we don't have a standard, it's that we've had multiple standards over what we think is the most basic of formats, with lots of hidden complications.