[mellosouls]

BBC say they have 'confirmed' it's Excel, though still a bit mysterious on how they've done that: https://www.bbc.co.uk/news/uk-54422505

Particularly, they reckon they were using the older XLS rather than XLSX format with the lower 65000 row limit - although that seems more like a deflection from the fact functionality limits weren't properly handled, tested or designed for.

[dang]

That article seems both more informative and more up-to-date, so we'll merge most of the comments from this thread into https://news.ycombinator.com/item?id=24689247. Thanks!