Yes. Clearly bad karma in itself is not enough for trolls and others who frequently get downvoted - there need to be some more tangible effects like comment hiding. This should have been discussed by the authors but I can't see that they did that (only skim-read the paper, though).

This interesting sentence from the abstract confirms what you say about downvotes being rewarding:

Interestingly, the authors that receive no feedback are most likely to leave a community.

Hence negative feeback is better than being ignored.