I was arguing against the idea that Amazon is unchallenged. I wasn't calling those companies startups.

There aren't/won't be many startups that compete with Amazon as a general retailer because it would take unthinkable amounts of time and money to have access to the inventory they have. It's like asking if there are any oil startups challening Exxon.

Yes, I'm not convinced either. They are neither startups nor appear to be competing hard. Jet.com was probably only good challenger which ran for sometime but I dont know what's their status now.