Microsoft is using Chrome to develop mobile applications (plus.google.com)

Yeah, but they can afford to do that now :). This is what you get when you are playing nice with all platforms.

IE/Chrome, Visual Studio/Sublime, ASP.NET/Node.js - who cares, just be a Microsoft developer and be happily productive.

Of course they are. I would be seriously worried of using M$ products if their developers still used IE for development. That said, I yearn the day that IE is happily buried in its own filth. No later than today I had spend 3 hours fighting unexpected IE behaviour. Sigh..

Mostly becuase is sad to say, but mobile developers only test in Chrome these days, so MS is practicly being forced to use it.

Is this surprising? I work at Microsoft and all development machines have IE+Chrome+Firefox installed. Our products, like everybody else's, are required to work on all browsers.