Yeah, it looks like I assumed the official doc was still the page I needed to check. I was able to find that now (although it was a bit confusing since those are two different things...settings vs controls). However, it's working now.
But now I'm wondering about something. None of the other emulators that use the six-button controllers require me to force the six-button settings in the Retroarch core first (or at least the Sega-based ones). With core remapping, I'm able to get them to cooperate. Would there be any benefit to turning on six-button in the other cores (where core remapping is already working fine)?