If we are asking the question:

What is it, Davis asks, that makes certain thinkers – Marx, Freud, Nietzsche – legendary?

The only thing that makes them legendary is the attention they got. The widespread sharing of their works, discussions, classes, expansion of their thoughts into diverse areas (as AdMan707 says, seeing Nietzsche in an ad class). They can be interesting or uninteresting, true or false, it is all irrelevant (for now). The only thing that elevates them to legendary is the fact that they got, and continue to get, an amazing amount of attention.

So instead of saying "they are legendary because they are interesting - say what the actual case is and then ask the secondary question, "why did they get so much attention."

This is a much easier question to answer and doesn't jump to the weird conclusions the author of this piece seemingly jumps to. There are a number of reasons something can get attention - maybe because it is interesting. It is easy to share or discussion. Maybe because it is true. Maybe because the font was 10 pixels bigger so more people read it. Some true items are interesting because they are true, even though the item itself may be inherently uninteresting.

So, if you want attention, you can do all sorts of things to get attention. You can shorten your writings to 100 words so more people will read it. You can make it colorful. You can lie or dumb down facts. You can say things are bad are good or good are bad and get people's attention. If your goal is to get attention, then you can make decisions on how to do so very easily.

If your goal is to create great work, explore a topic fully, write something amazing that you can be proud of, then you should do that and you should be proud of that. If it doesn't get attention, that is okay because it isn't your goal.

I think so many people get confused and do things for the wrong reasons. Don't go into academia for attention. If you want attention, become a master of meme-making or 100-word blog writing or acting or start a youtube channel. It's much easier. If you want to explore truths, then do that and don't erode your truths for attention.