Well, in motosports, the competition is not only between riders but also between team/constructors. A team needs to prepare a good bike for the riders so they can win a race and at the same time contribute to the constructors' competition. If Marquez or anybody else wins a race or championship, then the answer would be because of the rider and the bike quality. It doesn't make sense to assume that it's only because of the bike. Even if that's true, the settings of that bike or the quality of that bike come from their own research, trial and error, and so on.
Just to let you know is that each rider has a different engineering team.
The two drivers for example that are in the same team, actually is like they are not in the same team.
So, of course it is both the drivers and the bike but a huge portion of the success comes from the bike and the engineers that work on that bike.