I think you underestimate the amount of medical knowledge possessed by the current Healers. It takes the Yellows less than a minute after seeing Fire and Earth being used in a "Healing" weave to come up with plans to use them against heart conditions and so on. Which indicates a clear idea of the problems the current battlefield Healing couldn't solve, and also a pretty detailed picture of those problems.

While you're right that they don't have codified medical knowledge, especially at the sub-organ, tissue, cellular, and sub-cellular levels, they do have something that other primitive Healers in our world never had... Delving.

Take the case of Nynaeve delving Naeff. She doesn't describe the surface of the brain as the cortex, and doesn't give names to the deeper regions of the brain where she sees the taint-compulsion reach, but she can sense it, and Heal each of those individual regions. And she can regenerate various cell types, as she did when she regrew flesh and skin and nerves and blood-vessels in Rand's forearm when Semirhage burned it.

What Nynaeve doesn't have is the terms for these things. And she probably can't send weaves into the "pleasure centers of the brain" without delving, like Semirhage can. But from all evidence, her Healing is as fine and detailed as Semirhage's. Since she can actually purge specific poisons and microbe-based diseases with no issues, I assume that a lack of actual knowledge in biochemistry and microbiology is not a hindrance, due to delving.

None of this negates your point about why this technique was lost, of course. Nynaeve is an outlier in that she has extraordinary abilities in Delving and Healing. And women like her were incredibly rare even in the AoL. No way could this technique have survived the Breaking.