At risk of running afoul of HN guidelines: did you actually read the post?
The proposal does not add type checking to JavaScript. It adds type syntax. There is a huge, huge difference.
Their proposal is to implement a comment-like type syntax. These types would be totally ignored at runtime.
This isn't "run TypeScript in the browser"; this is "add syntactic sugar to vanilla JavaScript to get the development benefits of types without a build step."
Yes, JavaScript engines would have to implement a parser for this syntax. Yes, this raises the barrier to building a new JavaScript engine.
Does that mean JavaScript should add no new syntax ever again? No, of course not. So the question becomes one of cost-benefit analysis. And I think the TS team has submitted an extremely compelling argument for the benefits.
Your comment acknowledges the existing benefits of types-in-comments; think of this proposal as a significantly nicer way to write those.