I wouldn't say that assuming that "developers are good at everything, including all non-programming tasks" is old-fashioned. At the contrary I would call "old-fashioned" the idea of the caveman dev who only talk to their computer.
That being said, I agree that it's assuming a lot from developers, but that's a working recipe for a successful project. If you have another way, please do share it :)
This is not a working recipe for a successful project if you count good UX as part of the requirements. The modern way is full stack cross functional teams. You need UX designers working with developers, not to mention other domain experts depending on what you are developing.
I never said anything about "caveman dev." Software development is a challenging area, but being good at creating software does not make you good at other things. Some industries, like game development, have long had cross-functional teams where developers, designers, artists, writers, all work together.
In my opinion, any agile process that doesn't address UX design is outdated.
That being said, I agree that it's assuming a lot from developers, but that's a working recipe for a successful project. If you have another way, please do share it :)