I guess what they mean by nemesis is someone who is basically your peer, but sincerely trying to be much better than you. It works when they aren’t just your rival out to crush you, but someone much more “honourable” who wants to beat you the right way, maybe even share what they learn along the way. Rather than a nemesis, this is commonly known as a “great friend” and isn’t that much easier relationship to find than a mentor. In fact depending on the culture and industry, a mentor who isn’t all knowing but much more experienced and wiser than yourself is a more common occurrence.

I feel that people just bikeshed too much about these things when really all you need to do is be helpful where you can and don’t be an asshole.