The issue I see with that quote, and any Greg Quotes saying that “all” the Mangai were killed, is that they could easily be forgetcons; there is published material from the mid-2000’s that said some of the Mangai left before and (possibly) after the Toa-Dark Hunter War, as well as a Greg Quote from 2007 specifying the same thing. Both of those take precedence over the three quotes from 2014 which say/imply that all of the original members were killed.
It’s also worth noting that one of those three quotes specifies that they are only asking about the Mangai who didn’t leave after the War.