That is what I was thinking too? They have dominated Dakar since their first year. So much so that most the competition just quit. I don't know the rules for the Dakar but if it is similar to ISDE, they aren't going to be putting out a fresh bike every morning. Questions, Questions, Questions......
They do have mechanics that specifically tear the bikes apart and work on them all night after the race every day. A guy finished the Dakar this year in a "mostly stock" 640 Adventure without any major mechanical issues.
I'm also not saying that the 640 is hugely unreliable. It's main issue is vibration. The large tank sits tightly over the frame (and hoses/wires) and rubs on everything. Periodic inspections and preventative maintenance (I just got done cutting up an old heavy duty tube and wrapping and taping it around some radiator hoses for added protection) will go a long way on these bikes.
The main reason I say it is not a RTW world bike is the maintenance schedule. Also, the bike has 2 oil filters to change every time, has 3 drain plugs, 2 places to put back a pre-measured amount of oil, and then you need to bleed the oil system when you're done. Valve inspection / adjustments are scheduled to be done every 3000 miles. A new or rebuilt water pump will be needed every 10 - 14K miles (although this is a preventative measure based on other people's experiences with it going bad around this time frame). This all adds up to a lot of extra maintenance time and a lot of extra parts to carry around on your trips. Hell, I struggle with these issues on my vacations that are ~6000 miles.