Not saying I agree, but this is a completely valid viewpoint.
When I initially saw Typescript, I thought the point was to add features from strongly-typed languages and then transpile into Javascript. (IE, a more modern version of GWT, a Java to Javascript transpiler.)
The point of the article, though, is that Typescript works best when its extensions to the language can simply be dropped. That's clearly a "we've worked with this for many years and this is a big lesson from experience" statement, so I wouldn't discount it.
When I initially saw Typescript, I thought the point was to add features from strongly-typed languages and then transpile into Javascript. (IE, a more modern version of GWT, a Java to Javascript transpiler.)
The point of the article, though, is that Typescript works best when its extensions to the language can simply be dropped. That's clearly a "we've worked with this for many years and this is a big lesson from experience" statement, so I wouldn't discount it.