More than Outlook
The last paragraph of the article also mentions a couple of other issues
1) everybody else in Munich uses Windows, and that file compatibility is a problem.
2) getting missing functionality programmed is expensive.
Apparently a city council need a lot more than just Word, Excel and PowerPoint compatibility. Just a few examples: CAD files for buildings, financial reports from organizations that are subsidized, databases on all kinds of stuff to keep track off.
So when you try adding the missing functionality, which is possible because you have the source, then you find out that custom programming is expensive. Open Source was invented in organisations where having your own programmers around was the norm. But organisations do not employ their own programmers anymore in general. A city council s probably lucky still to have its own system administrators around, why on earth would they employ programmers then?
So you need to go contracting, and that is expensive.