> Even the echo example hints, that the created pipe has a name

Well, the echo example shows that the pipe is reified at /dev/fd/63 (or /dev/fd/62 for the other pipe).

That's not a named pipe. It's a file descriptor identified by the integer that defines it, accessed under the /dev/fd listing of all file descriptors.

So, to sort of answer your questions:

> How does diff know to read from those two pipes?

They're two different pipes. Diff takes two arguments. Each argument is one of the pipes.

> (implied) Is there such a thing as an anonymous pipe, in any context?

Not if you consider /dev/fd/id-of-pipe to be a name. The operating system lists them all there. But usually you would use a named pipe if you wanted to be able to know the name, so that you could find it again if you didn't already have a reference to it. If you jammed your finger into my chest, I would be there, and my physical form would block the passage of your finger analogously to how an anonymous pipe nevertheless exists as an entry (two entries?) in /dev/fd. But that wouldn't tell you my name in the conventional sense, even though it would be a valid and unambiguous way to refer to me.