According to the linked article, Their whole purpose was/is to “think 10,000 years in the future instead of quarterly”, so they only need 5 digits for that goal.

Yet they say things like "Our hope is that at least one of the eight headline languages can be recovered in 1,000 years". Come on, we can read text in almost any language from 1,000 years ago, and they didn't care at all about leaving durable texts. A couple of days ago there was this history about a norwegian carved graffiti from around 1,000 AD in Venetia, still readable. They should aim for 10,000 years.

You don't need anything else than one leading zero to have a non-ambiguous year number.

01998 will stay unambiguous even in the year 019980, and in the year 0199800, etc.