Garbage collection is completely different from these concurrency tools because these concurrency tools can be avoided by you while used by a library that you use, without you having to care. Garbage collection is like exceptions in that if a library expects garbage collection, you can't use it if you're not using that feature. Now you have to worry about the details of the library you use. That's why garbage collection is worse.
Also it's much more contrary to the spirit of the language. People already use threading libraries all the time, so having a standard one is not altogether absurd. The same is not the case with garbage collection.
The fact that you don't understand this, that you thought it was convincing to begin your post with "Do you feel the same way about, say, garbage collection?" makes it clear you don't have the C++ worldview to talk with Blunt about this issue.
The reason languages want threading as part of the language is for the memory model. The Boehm paper covers this in-depth. Libraries also need to be thread-aware to some degree, for example reentrant functions and thread-local storage issues.
Also it's much more contrary to the spirit of the language. People already use threading libraries all the time, so having a standard one is not altogether absurd. The same is not the case with garbage collection.
The fact that you don't understand this, that you thought it was convincing to begin your post with "Do you feel the same way about, say, garbage collection?" makes it clear you don't have the C++ worldview to talk with Blunt about this issue.