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They didn't study whether the reactive antibodies were protective, just whether they were there.

The thing the study establishes is that most adults have antibodies that will react to some parts of the pandemic virus. And then that's it. They didn't establish whether it was protective or not or anything like that.

We had a global pandemic. In that pandemic, a lot of people got exposed to Covid - exposed enough that they developed antibodies.

Antibodies are only produced if a pathogen replicates enough to trigger the immune system. So mere "exposure" doesn't do it; actual infection does.

In principle the absolute number of virions matters.

If you are exposed to 1000 versus 100,000, does one lead to an infection while the other indices some exposure response?

I would assume that most people have some sensitivity because they've been exposed to coronaviruses before. However this is not enough to quickly raise an immune response when the new variant shows up.