This used to be the case on Windows, but hasn't been for at least 10 years, and has never been the case on Mac OS X (using the historical name for clarity) or Linux. On Windows, the open-source NVDA screen reader is widely used. Furthermore, the hacks that Windows screen reader developers historically used to "find a way to make their product work", particularly intercepting GDI API calls (either in user space or in a display driver) to build an off-screen model, are not applicable to modern UI stacks. And the other major screen reader hack, using application-specific COM object models, was mostly only applicable to Microsoft Office and Internet Explorer. So you basically have to implement platform accessibility APIs to make your UI accessible. (If you use native controls, that's more or less done for you.)
Edit: BTW, I've been in the assistive technology industry a while, particularly on Windows. Feel free to ask me anything.
Edit: BTW, I've been in the assistive technology industry a while, particularly on Windows. Feel free to ask me anything.