Can quantum computers really compute a vast number of possible solutions simultaneously?
They are not constrained to stepwise calculations


As far as I know, all QC models are stepwise.


  Under the laws of nature, the ball spins either to the right or to the left, but never in both directions at the same time. The world of quantum computing is different: Here, the ball can revolve in both directions simultaneously


I think this is the source from which comes all this “QC is a massive parallel computation”.

It is not exactly wrong to think of superposition $\frac{|0\rangle + |1\rangle}{\sqrt{2}}$ as “both $|0\rangle$ and $|1\rangle$ at the same time”, but it’s confusing. It might make you think that both things are simultaneously available for arbitrary processing, which you can fully control. As if preparing a superposition is something like creating clones.

A more precise way to think about superposition is interpreting it as uncertainty. So, less confusing statement would be “neither $|0\rangle$ nor $|1\rangle$, until the setup forces it to be definite”.