Knowledge and technology require society to function. The post-Roman people were well aware of what the Romans built and even knew how to build them. What they did not have were the supply chains, large market, a consistent legal system, etc -- these things require a society in order to provide a place in which there can be an economy, that produces things that people need to implement their knowledge.

Do you actually think doctors / nurses would be able to get all they need through small trades / manufacturing?

What they didn't have where slaves to build them ;)

I think that knowledge and technology is much less dependent on "society" than we usually think. But of course it all becomes speculation in the end. The Romans in your example had constant assassinations of the most powerful people of their society. But I get what you mean, and of course you're mostly right.

I just don't see how supply chains for the medicines this man wanted could exist in a non-industrial society. These chemicals are complicated and require detailed production lines. It's not just the direct raw materials but also those necessary for the manufacturing in clean, sterile environments, etc.