Originally Posted by Metaluna

I could be wrong, but I believe the rule is that Di2 shifters will work with anything, but what you can't do is mix MTB and Road derailleurs, e.g. a road FD and mountain RD won't work together in the same network. But if you swapped in a mountain FD, the road shifters would then work fine (I'm not sure how well the mountain FDs work with road cranksets though). Since a 1x drivetrain has no FD, that apparently satisfies the no-mixing rule.