The errors in question has nothing to do with inherent flaws in Excel (beyond, perhaps, the difficulty of reviewing Excel code). The problems were in decisions made to exclude data, arbitrary "analysis" with no basis in statistics or common sense (19 - IIRC - years of data from England were given the same weight as one year of data from New Zealand), and an egregious formula error that simply ignored several rows of data.
beyond, perhaps, the difficulty of reviewing Excel code
This has always been one of my concerns when Excel is used much beyond 'electronic squared paper'. The interwoven data and code nature of the tool makes testing and verification more difficult - certainly not impossible but going against the grain of the tool. Excel is used by a diverse range of people, many of whom have limited software skills and could do with a tool that enforced the right behaviour.
Also I think this shows the need for openness when people make such important claims based on data. Particularly in this case where all the data and calculations could easily be zipped up and put on a website.
I don't think they were using Excel's weighted average function in this case but simply averaging filtered figures per country and then averaging the averages. This has no statistical basis, but it's not Excel's fault
I would suggest that in a situation like this, "the difficulty of reviewing Excel code" is even overstated. The direct dependencies of any cell are discoverable with trivial effort.
