differences between gcsfs and fsspec behavior when opening gs:// url paths

I would like to open a gcs path using fsspec's url resolver and then read it with xarray:

import xarray as xr
import fsspec

url = 'gs://ldeo-glaciology/bedmachine/BedMachineAntarctica_2019-11-05_v01.nc'  
openfile = fsspec.open(url, mode='rb') 
dsgcs = xr.open_dataset(openfile, chunks=3000)

this raises the following error:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-26-9f11dae45dd1> in <module>
      1 url = 'gs://ldeo-glaciology/bedmachine/BedMachineAntarctica_2019-11-05_v01.nc'
      2 openfile = fsspec.open(url, mode='rb')
----> 3 dsgcs = xr.open_dataset(openfile, chunks=3000)
      4 
      5 get_ipython().run_line_magic('time', 'dsgcs.surface.mean().compute()')

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/backends/api.py in open_dataset(filename_or_obj, group, decode_cf, mask_and_scale, decode_times, autoclose, concat_characters, decode_coords, engine, chunks, lock, cache, drop_variables, backend_kwargs, use_cftime, decode_timedelta)
    534                 "with engine='scipy' or 'h5netcdf'"
    535             )
--> 536         engine = _get_engine_from_magic_number(filename_or_obj)
    537         if engine == "scipy":
    538             store = backends.ScipyDataStore(filename_or_obj, **backend_kwargs)

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/backends/api.py in _get_engine_from_magic_number(filename_or_obj)
    113         magic_number = filename_or_obj[:8]
    114     else:
--> 115         if filename_or_obj.tell() != 0:
    116             raise ValueError(
    117                 "file-like object read/write pointer not at zero "

AttributeError: 'OpenFile' object has no attribute 'tell'

However, if I do the same thing with gcsfs, it works

import gcsfs
gcs = gcsfs.GCSFileSystem()

openfile = gcs.open(url, mode='rb') 
dsgcs = xr.open_dataset(openfile, chunks=3000)

This feels like a bug. And it breaks my mental model of how fsspec works. I thought that fsspec was just dispatching to gcsfs based on url matching. Help me understand why that is not the case.

xref https://github.com/pydata/xarray/issues/4591, which helped me discover this (but is about something different)

Start with the fsspec.open(url, mode='rb') entry point and compare its returned OpenFile with the object returned by gcsfs.GCSFileSystem.open. Check how xarray.open_dataset consumes each object, then add or update a focused regression test if the project contains one for this behavior. Done means the differing interfaces are explained and the reported gs:// workflow has a clear, tested outcome.