I'm pretty sure that was just a publicity stunt for him. There is only one black FXX without a stripe, and that one belongs to Schumacher, so I guess he (or Ferrari) didn't want anyone else driving it.
Schumacher isn't the Stig... He may have been "the Stig" for that day or at least the taping of the Ferrari FXX - which as Gary pointed out was his own FXX (there were only 30 made and it's unlikely that anyone would "loan" their uber-rare, 2.5 million dollar Ferrari to anyone, even the Stig to drive). Plus, up until '06 at least, I highly doubt that Ferrari would just let him run off to England whenever necessary while still racing... And he's been fairly busy since then.
If anything... It seemed fairly obvious that he was simply there to plug the new drunk driving campaign he's doing with Bacardi. Not to mention being a typical TG stunt in order to quell all of the media crap over there which has been surrounding the Stig's identity as of late.
The Stig is many people, depending on who they can get and where in the world they're filming at the time. His height and weight fluctuate wildly from show to show, so it's obviously been lots of different people over the course of the show.
As AMGoff said, this was just a clever way for Schumi to plug his "Don't drink and drive" campaign while giving Top Gear a big ratings boost. That said, the FXX doesn't belong on the power board since it isn't a street car. The Ascari qualified only because of the hydraulically raising front end that allows it to clear a speedbump, but the FXX has none of the gear required for a street legal car. By Top Gear's own rules, that disqualifies it from the Power Board.
Great lap, but the Ascari and Gumpert are still the kings.