We’ve been mapping the title attribute to aria-label and it looks like macOS’s VoiceOver ignores it.
What I’ve done is to map the label attribute (which is the visible text) to the aria-label attribute. A quick search shows up that 268 d-buttons have labels and only 69 have a titles, so other parts of the interface will also be improved. 
https://github.com/discourse/discourse/commit/31ffcf989ce63e451baaed7deb61bf3e3126f197