I didn’t actually assert 29ers are better at tamer courses, just that that’s where they first seem to have come to the fore, especially if you appreciated the benefits of a CX bike and knew how capable they are off road.
I actually think 29ers are very capable which is why I’m vaguely surprised the UCI didn’t protect 26ers from the disadvantage.
Enduros are generally tamer courses though, which is fair enough if you’ve got to link the decent trails with fire roads or even tarmac, and of course if you’re riding further on a circuit you probably don’t want a course that wears you out in 1.5 hours.
While 29ers may have been around before disc equipped CX bikes (although these also started appearing more than 10 years ago), I’d hazard a guess the earliest 29er riders had ridden both MTB and CX before 29ers came about.