I don't believe it's coming out in January 2020, I'm much more inclined to believe January 2019. It's already been in production for a year and a half, and they've got another half year before January 2019. Two years is more than enough. Even averaging an episode a month (and that's being VERY VERY VERY conservative since most anime average an episode every 1-2 weeks), they'll have had enough time to create 24 episodes worth of content. And even if Murata hasn't finished the Monster Association arc, it HAS been finished in the web comic so they're not working blind and a very conversations with ONE and Murata and bam, they can fill in the rest easily enough.