Re sort-by: the point there was to show how Dommy gets for free "features" that have to be added to jQuery and contribute to its code size.
Re one/map: you can still do operations on sets of 0, 1, or any number of elements, without knowing how many, by always doing map. There's nothing to keep track of, everything always only works on one object, so you're explicit about whether you're operating on exactly one object, or whether you don't care how many objects you're operating on. The latter is just as convenient as jQuery; the former is safer and deliberate. In jQuery, on the other hand, you have to keep track of what works on one object and what works on multiple objects: .text() and .text('a string') are asymmetrical, for example.
Re one/map: you can still do operations on sets of 0, 1, or any number of elements, without knowing how many, by always doing map. There's nothing to keep track of, everything always only works on one object, so you're explicit about whether you're operating on exactly one object, or whether you don't care how many objects you're operating on. The latter is just as convenient as jQuery; the former is safer and deliberate. In jQuery, on the other hand, you have to keep track of what works on one object and what works on multiple objects: .text() and .text('a string') are asymmetrical, for example.