On could argue that any explanation of what happened to the Valiant
is only as good as Spock's interpolation, which itself is a function of the condition of the recorder's memory banks.
In that spirit, maybe the barrier was the storm; the barrier does resemble a storm; well, it does more so under the original effects than under the "remastered" ones, at least. A maxim that a trope shouldn't be repeated twice, when once will do, would lend addition credence. Anyhow, maybe the recorder's memory was so garbled up, that it read that the storm was what threw them clear, rather than whatever it was that really did.
I could buy that, except for a couple of things.
First, if we throw out too much of what Spock says as inaccurate, then that basically defeats the whole purpose of having the dialog in the first place. The episode invests quite a bit of time in delivering it to us. We're not being thrown a red herring, we're being thrown a bone, in that dialog.
Second, Spock indicated when he found garbled data. Plus, he's Spock,
man, the second of our two heroes. So, we have to believe that what he told us was a pretty accurate account of events, as they were recorded. However you slice it, Spock said the storm is what threw the Valiant
out of the galaxy in the first place. And, he differentiated between the magnetic space storm that swept them to the edge of the galaxy on the hand, and the unknown force that mutated the seventh crewman on the other. It's therefore reasonably clear that the Valiant
crew made the same differentiation.
So, I can't accept an identity between the magnetic space storm and the barrier. For what it's worth, I'd thought of that possibility, too, basically because of the economy of tropes problem that I mentioned. But the other reasons not to believe it far outweigh that possibility, I think. We just have to accept that the galaxy is a stormy place, at least at certain times and places.