I didn't test it. You're right, I'm simply mistaken.

Here's a better (but awkward) illustration of the problem:

  :~$ python3 -c 'import sys; import shutil; shutil.copy(sys.argv[1], "foobar")' <
  (echo foo ; sleep 1 ; echo bar)
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/opt/homebrew/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/shutil.py", line 427, in copy
      copyfile(src, dst, follow_symlinks=follow_symlinks)
    File "/opt/homebrew/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/shutil.py", line 257, in copyfile
      raise SpecialFileError("`%s` is a named pipe" % fn)
  shutil.SpecialFileError: `/dev/fd/11` is a named pipe
  :~$ python3 -c 'import sys; import shutil; shutil.copy(sys.argv[1], "foobar")' =(echo foo ; sleep 1 ; echo bar)

Notice the difference:

  :~$ echo =(echo foo ; sleep 1; echo bar)
  /tmp/zshbusU5D
  :~$ echo <(echo foo ; sleep 1; echo bar)
  /dev/fd/11

/dev/fd/11 is a named pipe, whereas /tmp/zshbEF1D9 is a plain file. There are situations where named pipes fail, where you actually do need a plain file, and I can't think of a single one offhand. :)

"I've run into it in the past" is all I can say. And the above snippet at least gives one specific example of a hypothetical case.