It is a concept that's difficult to explain to non-German speakers, admittedly. Plus in the UK you have the additional aspects of old buildings being draughty (whereas an Altbau is usually solidly built and non-draughty) and the surprise at Germans renting a place voluntarily, not out of necessity.
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(This is for Vienna but the architecture is similar to the German.)
https://buehrer.de/architektur-der-gruenderzeit/
This could help to fill the gaps in his knowledge.
Maybe you have been in Leipzig-Plagwitz years ago? It is a Geünderzeitviertel.