I know what he means.
Paganini was considered a wonderful violinist, but no recording of his work exists, so how can we put him in historical context as a "great" musician.
I guess we have to base it on reports of their work, what other musicians and critics said about them.
Same with some early Jazz musicians.
Some early New Orleans muscians were never recorded so we only have the word of people who were in New Orleans at the time to say how good they were and how they compared with people like King Oliver and Louis Armstrong who WERE recorded.