I could in a post-apoc world, make bikes and tell people how to make bikes, so that point is null.
You could if you were who you are today, yes.
But if you were born in Fallout's universe? where the only bike you've seen may be scrap metal? Where people have had more important things on their mind - like food and shelter- than getting some old rustbucket going? Where you never had that first bike your parents gave you? Where they never taught you to ride and likely never learned themselves?
KNowledge, lost completely?!?!?!? nope
You mention that people know how they work because they were taught.. did everyone on the planet that previously used a bycicle die. or can we assuime that there would indeed be people that had used a bike that survived the great war?
there are people still, right?
assuming that the statistical improbablity of everyone who had ever used or seen a bike had died in the war, i still think people would figure it out.
The morotcycles in game would be a pretty clear example how a crank, chain and gear assembly worked.
the wastelanders wouldnt really have to be taught how they work, just see an example.
its essentially a pulley.. not complicated, and there are cexamples of that type of tech in fallout..
Given that there are also many skill books in game, i would assume that there are a good deal of peole that can put together how a bike works.
Although the lack of evidence doesn't mean there's definately no bikes, there is no evidence of a working bikes existence either. the presumption has to be on their non-use, rather than their use.