The traditional way is vertically, indeed, but on occasions you had to write horizontally, which was then usually also done right to left (can still be seen here and there, as I mentioned in my previous comment).

> The traditional way is vertically, indeed, but on occasions you had to write horizontally

There's not really a distinction. Chinese was written in columns from right to left. When the columns were one character long, they were still columns.

To distinguish horizontal writing from vertical writing, you'd need a writing sample that had multiple columns and also multiple rows. (Because, as noted above, there's no way to tell the difference between 1 row of 5 characters vs 5 columns of 1 character each.) As far as I'm aware, traditional Chinese writing is fully accounted for by the theory that it was always written in columns and never in rows.

On the other hand, the Taiwanese newspaper isn't making a great case: the "mixture of approaches" used in the headlines are a couple vertical headlines going top down, and several horizontal headings, including the primary headline "Michael Jackson、arrives!", which all go left-to-right. The body text in the infoboxes in the top left and right corners of the page also goes from left to right, though it appears to be true that the article text is in columns. The name of the newspaper (top center) is also printed left-to-right.