If it was a simple average it would have been mangled. This is a deliberate fine-tune of the model to the particular style. For fine-tuning you need some type of input, if generated or human made doesn't matter.

The reason why they fine-tuned to this particular style is unknown, but they might be:

• To quickly and not as expensively produce and release a usable model in the then more heated, now slowing down competition for better and better models
• To reduce the model size as there is no need for lots of data on multiple styles.
• To give a distinct comics style to their model, so to make people associate the image to their model instead of OP

There is nothing wrong with fine-tuning, and it is very often necessary to have the output not be gibberish.