If you're talking about TypeScript, I don't believe that mantra is applicable when the extensions are open source, liberally licensed (Apache License), cross-browser, and developed in the open. There's nothing proprietary or exclusive about it.

To extinguish, you introduce incompatibilities to make the life of people outside your inner ecosystem (compared to the greater, standard-compliant ecosystem) harder. But since TypeScript compiles to JavaScript and interfaces with plain JavaScript wells, there is really no "extinguish" part here.

Also, the EEE accusation doesn't really apply to open source projects. EEE is bad since after the proprietary product supersedes ("extinguishes") an open product, the community loses control forever. However when an open product supersedes another, the community always have the choice of forking the project to take back control.

Hard to say. It didn't seem like Microsoft had a good strategy for modern web development (javascript/rest/etc.) at least from the client side. They had webforms/MVC but you pretty much had to use client side controls from other sources, like Telerik, if you wanted to do something modern. One assumes that tooling with javascript would be difficult for them to add in Visual Studio, say as compared to the tooling offered for C#. So moving to a typed language allows them to have nice Visual Studio tooling as well as picking a path for modern web development.

Seems more like AtScript was an 'embrace extend' attempt on TypeScript, which has been somewhat rebuffed...