The blog post is a response to "software folks" (DMR declines to offer specific examples) who are "shocked" that JPMorgan used Excel to create risk models for certain portfolios.
A precis:
Geeks are shocked that people are using Excel for things, but they shouldn't be. (First section)
Spreadsheets began with VisiCalc and Lotus 123 and have been popular for a long time. ("Birth of the spreadsheet")
Excel is basically just agile development for business management. ("The business world runs on Excel")
Excel can be used for other things too, such as UI wireframes, attaching screenshots to emails, and making numbers line up. ("It’s not just a spreadsheet")
Excel itself never contributes to or causes any problems, and these problems can be easily solved by using better tools. ("What can we do about it?" and "Don’t hate the spreadsheet, hate the game.")