The tone of the article is obviously ironic and exaggerated for humour. The joke is "those crazy foreigners still believe the Earth is flat in the year 1900, haha, I bet next they'll be burning teachers at the stake", which is a bit mean to the fine folk of Portsmouth but for a local paper in Minnesota reporting on events across the Atlantic some stretching of the truth in service of a laugh is acceptable.

What it proves, then, is that believing the Earth is flat was already well-refuted enough to be the butt of jokes by the year 1900.