People keep saying this; however, as stated, it is false. It is not impossible to make any kind of informed guesses about historical descriptions of medical conditions that were not (well) known at the time. Isn't this "throw up our hands/how can we know anything" response the very opposite of the practice of history, anyway?

As with almost everything we ever know anything about, we can often improve our state of knowledge about these things, but almost never gain an absolute certainty of understanding. But learning something is not the same as something being "impossible" to learn anything about.

Based on memories from grad school and some basic common sense, here are some points about learning about historical medical conditions:

• In some cases, a condition has extremely specific symptoms, allowing a reasonable historical description to be reinterpreted in light of new knowledge. This is almost certainly the case with schizophrenia and obsessive-compulsive disorder, for instance--some historical descriptions seem specific and clear enough to rule out other possibilities (in particular cases) with a reasonable degree of certainty.
• Some conditions leave physical evidence that can be reinterpreted with modern knowledge (e.g., osteoporosis).
• Sometimes the modern understood cause of a medical condition allows strong statements about historical epidemiology--for example, what we know about the action and heritability of sickle-cell anemia allows reasonable guesses about historical prevalence in some groups.
• In some cases, hypotheses about how social or environmental changes might have impacted the rates of medical conditions can be tested in smaller societies that have not yet undergone the changes we talk about. Examples include hunter-gatherer groups in which hypotheses about the effects of post-agricultural or post-industrial societies can be tested.

I'm sure there are other methods, especially with the amazing advances in population genetics and epidemiology that seem to be coming at an alarming rate.

Simply saying "we can never know because that was history" (I oversimplified a bit) is not an accurate depiction of the situation. We very well might, as far as I know, be able to get reasonable estimates of the prevalence of certain allergies in pre-1900 populations.