if a pandemic reduced today's civilization to the state in which humanity existed for the majority of its history
Why would it? A pandemic wouldn't destroy knowledge or technology.
Consider Black Death -- it reduced the population of Europe by something like a third, I think. Was it a big deal? Sure it was. Did it send Europe back to the time when it was populated by some hunter-gatherer bands? Nope, not even close.
We have a lot of systems that depend on one another; perhaps a severe enough pandemic would cause a sort of cascade of collapse. I'd think it would have to be really bad, though, certainly worse than killing 1/3 of the population.
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