It may not be inherent but IMO that is just hair-splitting, the important part is that it is there (also FWIW i had PostThreadMessage in mind, not PostMessage) and is very convenient to use. I've used it a bunch of times whenever i worked on Win32 tools that i wanted threading by sending commands to other threads to do stuff (e.g. enumerating the directory structure for an asset browser in a game editor at the background to avoid stopping the UI) and having them reply back with messages about the results.

And honestly i never had any bugs with that, if anything i've found it the easiest approach to understand when it comes to inter-thread communication.