You're missing some information.His donations did a lot to build hype during events like Hidden Cup 2 so it seems rational people might be curious about what he might be doing now. He is verified on Twitter so not exactly a private figure at any rate.
What does his employment status at Microsoft have to do with him supporting AoE2 tournaments?
Matt Salsa is the original founder of Mixer (then called Beam) that was acquired by Microsoft. He kept heading the Mixer division of Microsoft (I guess at least from a technical standpoint) for a while.
To be honest I don't know if he heard of AoE 2 because he was working at MS or because he played it as a child like many of us. However he was involved in some AoE 2/Mixer events such as Viper moving to Mixer temporarily. To be clear though, I am not implying he donated because of he was at MS. Pretty sure he did that simply because he enjoyed the game and its personalities.