The keyword here is 'much higher'. It is of course not guaranteed. As a developer, if somebody already done the trouble shooting or even root cause finding for you. You are more likely to fix the bug first because it will only cost you a little time compares to other completely non-investigated error report. And that also benefit the users because they got the bug affecting them fixed earlier. This don't happen in closed source softwares.

Plenty of funded proprietary closed-source software are plagued with bugs too