Because I've heard him say he started booking in 1999 and then I've read that he was booking during the Sting/Goldberg non sanctioned title match, that lead to the 64 man tournament.
By 2000, he was out and we got the rotation of Russo, Ferrara, Kevin Sullivan, Terry Taylor, Johnny Ace, Bill Banks and other random guys.
And for the record, if you're talking about Thunder in 2000, the title was stripped from Sid, Nash gave it to himself and Sid beat him for it later that night. By my math, that's only 2 actual changes (Sid to Nash, Nash to Sid), which has happened A LOT. It's also well known that Kevin Sullivan was booking at this time, seeing as how him being named booker led to Benoit & company jumping ship, Sullivan having a feeling that would happen, tells Sid when Benoit puts him in the crossface, make sure his foot is under the ropes before tapping out and that was a week before the Nash/Sid title changes. And if i'm not mistaken, Sullivan remained head booker until Russo & Bischoff returned in April.
I don't believe Nash had any real booking power in 2000. Late 1998 and early 1999 then yes he was apart of the booking comittee and helped put himself in the match against Goldberg at Starrcade to end the streak. 2000 had a lot of leaders/bookers but I don't think Nash was one of them