I read the entire article, and I love how virtually every comment here is what Dan wrote about.
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Ballmer, or Microsoft under Ballmer, had to have been laying the groundwork for Azure, TypeScript and VS code before they took off under Nadella.
More specifically: Nadella under Ballmer laid the groundwork for those things.
Dan lists three big wins for Ballmer: Bing, Azure, Office365.
Nadella led Bing and Azure. Not sure where Office365 sat in the org chart, but even if he wasn't managing it a _lot_ of services at Microsoft at that time relied on technology that was developed in Bing and Azure.
He was promoted for good reason.