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Because there's probebrly only 1 real route.
There's no point keeping it private really. MB knows a lot of other people and they will have no doubt given him their routes, especially Jasmijn and others.
Time trialling is a good community, and people want to see the records broken. I doubt anyone would not share their route with another community member, or go out of their way to help.
So why has MB published his route if he’s going first? ;)