Not sure why it took so long because the account has been around for quite some time. TLC's twitter account has issued previous denials of any real Roloff twitter accounts, but they did finally confirm that:
http://twitter.com/mattroloff is the real Matt Roloff. Although he doesn't use it...
"LITTLE PEOPEL BIG WORLD fans: just in time for pumpkin season, Matt Roloff is on Twitter @mattroloff! Season 5 starts Oct. 12!"