I keep telling people that a patent isn't a measure of the quality of the idea, and certainly doesn't mean anything about the marketing claims. Indeed it is much easier to patent a stupid idea: not only is it likely that nobody has published the idea before (no anticipatory prior art), but there will be no end of people saying you should never do anything remotely like the idea because it is stupid (the mass of the prior art teaching away from the idea is a very strong defense against the examiner saying the idea was obvious). Honestly this is a bit of a strange situation because people have come up with similar dumb ideas, but just had not published this combination. The examiner likely was hamstrung and unable to say the missing specific bits or shapes were obvious because then they would run smack into the realm of "this is an incredibly dumb idea, don't do this sort of thing ever".
Also, this thing functions as a crank just fine. A heavy, expensive, ground-clearance killing crank, but a crank all the same. Pedaling forces get transmitted to the chainrings, in accordance with normal laws of physics and leverage. It isn't doing any magnification of pedaling forces or anything, but the courts have held that the bar you have to clear for utility is pretty much "has at least one disclosed use to do something more than sit there, even if it does so in an unreliable fashion". They say it is a bicycle crank, that is a believable use, so that's good enough to clear the bar. If they instead had only said it was a cancer cure, then they would be lacking utility and would be rejected on that ground.