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Using Excel is strange, so using columns really isn’t a massive leap. It is rows as per other articles, there aren’t even that many tests available to go over 1m.

Modern versions of excel (after office 2007), using .xlsx files, have a 1,048,576 row limit. But older versions of excel (before office 2007) used .xls files and had a limit of 65,536 rows.

I could easily see them exceeding this limit, if they were using an older version of excel.

Alternatively, even the new versions of excel enforce the 2^16 row limit if working on a .xls file instead of a .xlsx file.

The article says they went over the row limit so the later.