Yes, it is monopolistic - some call it technofeudalism, because Amazon owns the "land" and extract land rent out of it - with questionable service in return.
The argument about Amazon requiring cheaper prices is true in theory but in practice not strongly enforced. I just bought a kettle on Amazon instead of from another seller who had it 2 euros cheaper because I combined the purchase with other stuff that the seller didn't have. I have readily told people that some items they were buying could be found for cheaper on AliExpress, but they didn't care because they wanted the convenience and fast service. By the way, many of Amazon's suppliers sell the same product cheaper on AliExpress and other marketplaces; that completely kills the argument in the first place.
Amazon is successful because other companies were complacent and ignored the upcoming internet economy. Amazon built a very complex logistic system that took a very long time and a lot of investment to build. Now that they dominate, the lazy competitors cry about monopoly. In my country (France), the traditional supermarkets are just about now coming in with alternatives to compete, and they are very far off the mark for both service/selection and often price as well…
The MFN principle is enforced - but only for the same reseller. The same product may be sold in another channel under another reseller, Amazon cannot touch that. AliExpress is surely outside of that scope. Also, EU managed to "convince" Amazon to stop MFN in principle in EU countries, practice is different (they don't enforce it, but give credit breaks) - it's still being enforced in USA and subject to legal challenges: https://www.johnstonclem.com/news-insights/amazon-antitrust-...