Dissassociation of atoms in molecules by radiation is only the first half of the process. The question is what is the equilibrium result of reassociation of the freed atoms into molecules of H2, O2 and H20. Presumably, that could have some ambient temperature effect.
But, considering that so many of the astronomical bodies that we see have large amounts of H20, even rubble pile asteroids, and those presumably formed in space and have been subjected to large amounts of cosmic rays. it seems pretty dubious that the interstellar environment could so completely convert all water into molecular hydrogen and molecular oxygen.
So, any paper that proposes that should at least be offering some evidence that is what happens. In this particular issue, that evidence could be (could already have been) developed in a lab here on Earth. So, before somebody publishes a paper claiming that effect, they should have included something supporting that claim. If not, then the paper should be questioned, not just believed because it was published.
As for "gravitational slingshotting" using the Sun: Yes, it is not something that would work for space craft originating from Earth to gain energy for reaching orbits farther from the Sun. However, it is not out of the question that space craft that originated from a different star system could use the difference in the velocities of the two stars to gain some energy to get farther from the center of the galaxy. But, more likely, the Sun could just be a conveient way to change course to go on to another star system. That is pretty much how we are doing tours of asteroids and Keiper Belt objects with our own spacecraft. We do fine tune trajectory adjustements to make the encounter with the latest target result in an "open orbit" around that target that results in a trajectory that points to the next target. Not a "slingshot" but more like an elastic collision. Kind of like a game of billiards, where the obect is to make the cue ball touch both of the other 2 balls on the table by making the cue ball change direction with first contact to go towards the location of the second ball. Presumably an advanced civilization could use that strategy to send a probe to look at more than one star.
I am not saying that is what Oumuamua is - just explaining the possibilities and the proper logic.