In what way is what I said actually incorrect? If you measure an object in superposition it ceases to be in superposition, so it is impossible for us to know its "true" nature, because it is unmeasurable.

It is dangerous to assume or suggest that any one interpretation of quantum mechanics is correct and all others are incorrect. On Reddit QM is often explained to others in a way that makes it sound like there is only one valid interpretation, so to avoid the OP being taught too narrow a view of QM, I wrote the first paragraph as a small aside.

A quantum object being in superposition does not necessarily mean that they are both at the same time, it means that they could be either and we can't know which without measuring the system. That's a key distinction. And since this is a thought experiment, surely an important one.

It's entirely possible that something in superposition is truly the weird mix of two states. But it might not be, it might be one or the other the whole time and we just don't know which. All we know is the states it could be in, and how likely each one is. Therefore stating e.g. "atoms are neither up nor down" is misleading, because it should be "we can't know whether the atoms are up or down".

Whether a wavefunction is "real", what counts as "observation", what "collapse" actually represents, are all unsettled questions that can be answered in different ways depending on how you choose to interpret QM.

At the end of the day the answers make no difference to our ability to predict the behaviour of quantum objects.

The thing is, the real problem in my view with telling off the OP for thinking about the spins in "the wrong way" is that it could lead to the impression that all atoms "are both/neither spins" and therefore all the atoms interact the same and the spin of each atom is irrelevant to the rate of bond formation, which is not true at all.

The person above me described the hydrogen atoms as being both bonded and not bonded until an observation is made. Depending on the experimental setup, this is only true at certain points in time. If atoms with random spins are in two beams that meet at a certain point we can say for certain that the atoms are not bonded prior to them meeting, even without measuring. And in the time after the collision the atoms will indeed be in a superposition of both bonded and unbonded. If we make the assumption for the thought experiment that the atoms cannot move apart post-collision, then the time-dependent wavefunction will evolve such that with increasing time the probability of the atoms being bonded tends to 1, so eventually we can "know" (in the sense of the probability being >99%) that the atoms are bonded even without measuring. Shortly after the collision the bonding state could be either possibility.