Why are you assuming that Engels and Lenin made the "correct appraisal?" To me, these examples illustrate that they both used fiction inappropriately. Just because Engels and Lenin are notable humans doesn't make them infallible.

Imagine you were having a discussion with someone about some topic, and your discussion partner said:

> I have learned more from [some novelist] than from all the professed historians, economists, and statisticians [who study the topic].

Would you really consider that a well-reasoned opinion?